When a decimal-looking number is not a decimal: 4.5 hours, 5.5 overs

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What the video covers, what to say before it, where a class usually goes wrong, and what to set afterwards. An account is free, and it opens every chapter of every book.

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What to assume they know

Extending Indian place value to the right of the point — the point, and what the places to its right count (Part I, §3.4)
Why measurement units are built in tens — why the metric and money units are built in tens (Part I, §3.5)
Telling the time on a twelve-hour clock, and the relation of an hour to sixty minutes
Taking a fraction of a quantity, as in a third of 60
The idea of a unit divided into a number of equal parts other than ten

What they should be able to do

Say what a digit after the point counts in a genuine decimal number
Work out, for a quantity written in a non-decimal convention, what one tenth of its unit would be worth, and compare that with the convention actually used
Convert a duration written in decimal hours into hours and minutes
Convert a length written in decimal feet into feet and inches
Explain what an over-count such as 5.5 means, and which denominator is in force
Identify, from an everyday quantity, whether the notation after the point is decimal or not, and say how you can tell
Give a further example of a decimal-looking number that is not one

Where it usually goes wrong

"4.5 hours means four hours and five minutes." It is the first candidate the chapter rejects, and it is the commonest reading among students who see the digits and think of a clock face.
"4.5 hours means four hours and fifty minutes." The second rejected candidate, and it comes from treating the digit after the point as a count of tens of minutes.
"2.5 ft means 2 ft 5 inches." The whole point of the door story. Five inches is not half a ft, and the two notations differ by an inch.
"5.5 overs means five overs and five tenths of an over." In cricket the over is cut into six, so the second digit counts balls out of six, not tenths.
"So the careless reading works for cricket." It gives the right number of balls here by coincidence, not by reason, and the same carelessness gives the wrong answer for the bus and for the door. Show why it works and then show that it does not generalise, rather than leaving a student with a rule that happens to have worked once.
"A number with a point is always a decimal number." The section exists to refuse this. What makes a number decimal is that the unit was cut into ten, and that is a fact about the situation, not about the printed mark.
"You can add times as if they were decimals." Two durations written as 2.45 and 1.30 in the clock sense do not add like decimals, because the sub-unit does not carry at ten. This case is an added extension of the chapter's argument and should be flagged as such.

Questions to check understanding

Convert a duration written in decimal hours to hours and minutes, and back
Convert a length written in decimal feet to feet and inches, and back
Given a score such as 5.5 overs, say how many balls it stands for and why
Decide whether a given everyday quantity uses decimal notation or not, with a reason
Say what one tenth of a named unit is worth, and compare it with the sub-unit actually used
Produce a further example of a decimal-looking quantity that is not decimal

Examples worth working on the board

The bus message (Part I, §3.8, p.77). A girl is told the bus reaches the station 4.5 hours after midday, and is offered three candidate arrival times: 4:05 p.m., 4:50 p.m. and 4:25 p.m. The chapter rejects all three.
The working the chapter prints (Part I, §3.8, p.77). Split an hour into ten equal parts; each part is sixty minutes divided by ten, so six minutes; five of them make thirty minutes; the bus reaches the station at 4:30.
The door (Part I, §3.8, p.77). An opening measured at 2 ft 5 inches is passed to a carpenter as 2.5 ft. The chapter supplies twelve inches to a ft, so half a ft is six inches, and the carpenter builds a door of 2 ft 6 inches. It will not close.
The cartoon strip (Part I, §3.8, p.77). Checked, and read from the printed page. The two speech balloons do come through the extraction; nothing of the drawing does. Three panels: a girl looking at a door that will not shut; the girl and the carpenter arguing, her speech balloon insisting the width was to be 2.5 ft and his replying that it is; and a third panel of the carpenter holding a tape against the frame, with a bystander clutching her head. It is a joke about two people using one notation for two different agreements, and it is worth working through rather than summarising.
The cricket score (Part I, §3.8, p.78). A display reading "Overs left: 5.5", with two candidate readings offered: five overs and five balls, or five overs and three balls. The chapter supplies one over as six balls, writes the quantity as five and five-sixths of an over, and settles on five overs and five balls.
The trap in that example, to bring out. A student who ignores the point altogether and simply reads the second 5 as a count of balls arrives at the printed answer. A student who reads the notation as a genuine decimal gets a different one, because five tenths of six balls is three. The three-ball reading is exactly the distractor the chapter prints. This observation is added here; the page states the answer and the reason without commenting on the coincidence.
The closing prompt (Part I, §3.8, p.78). A boxed question asking where else such notation turns up. Sports scores, clock times, and lengths in feet and inches are the section's own three; the explanation may collect others, but should not present them as the chapter's.

Figures to have open

An hour bar cut into ten, with a clock face beside it, so six minutes can be read off rather than asserted. Standard schematic, and the topic's key graphic.
Two overlaid grids for one hour, one in ten parts and one in sixty. Standard schematic; not in the textbook, and the clearest way to show why the clock tempts a wrong reading.
A door frame and a door, differing by one inch. Standard schematic; the textbook's cartoon on p.77 covers the same ground and may be redrawn instead.
An over drawn as six balls in a row, markable in two ways. Standard schematic.
No photograph is required, and nothing needs to be lifted from the textbook.

Where this sits in the book

NCERT Ganita Prakash, Class 7, Part I, printed Chapter 3 "A Peek Beyond the Point", §3.8 More on the Decimal System, and its second unnumbered subheading Deceptive Decimal Notation, which opens on p.77 with the bus and the door and continues onto p.78 with the cricket score and the closing prompt.
Backward pointers: Part I, §3.4, p.62, for what the point actually declares; Part I, §3.5, pp.64–69, for units that are built in tens and therefore behave.
Forward pointer inside the same section: Part I, §3.8, p.78, on how the separator itself came to be chosen, which is the same question — convention rather than necessity — asked about the mark instead of the unit.