61
Review problems
Textbook: Part 2, lesson 61, pp. 97–99
GoalReview: decimals, rounding, motion and fraction problems.
New words
decimal · o‘nli kasrrounding · yaxlitlashcatch-up speed · quvlab yetish tezligiperimeter · perimetr
Explanation
In the review we practise reading, comparing, adding and subtracting decimals. In addition and subtraction the points are written under each other and the answer keeps this order. For rounding we look at the first dropped digit: from 5 to 9 add 1 to the previous digit, from 0 to 4 leave it unchanged. Catch-up problems use the difference of speeds: the later body reduces the initial gap at speed v₁ − v₂. In fraction problems we find a part of a number, or the whole from its part.
Worked examples
Calculate: 9.8 + 3.45 − 2.6 = 13.25 − 2.6 = 10.65. Rounding: 10.65 ≈ 10.7 (to tenths), because the dropped digit is 5.
A cyclist set off 2 hours earlier at 15 km/h. The gap is 15 · 2 = 30 km. A motorcyclist chasing at 45 km/h catches up in 30 : (45 − 15) = 1 hour.
Class activity
“Review cards”: write 6 decimals on cards, compare them in pairs, then round them to tenths and arrange them in increasing order.
Practice
1
Calculate 12.6 + 8.45 − 5.15.
15.9
2
Round 7.496 to hundredths.
7.50
3
A lorry left 2 hours earlier at 40 km/h. A car chases it at 60 km/h. In how many hours does it catch up?
4
4
A rectangle has width 12 cm, which is 3/4 of its length. What is its area (cm²)?
192