124
Solving cost problems (practice)
Textbook: Part 4, lesson 124, pp. 43–45
GoalHow the cost changes when the price or quantity changes; comparing prices; multi-step problems.
New words
increased · ortdicomparison · solishtirishcheap · arzonexpensive · qimmat
Explanation
If the quantity stays the same and the price grows several times, the cost grows the same number of times. If the price stays the same and the quantity grows several times, the cost also grows that many times. To see which of two packs is cheaper, we find the price of 1 kg (or 1 piece) of each and compare.
Worked examples
4 kg of pomegranates at 10 000 sum/kg cost 40 000 sum. If the price doubles to 20 000 sum/kg, the cost is 20 000 × 4 = 80 000 sum, which is 2 times more.
4 kg of rice cost 48 000 sum and 6 kg cost 66 000 sum. Prices: 48 000 : 4 = 12 000 and 66 000 : 6 = 11 000 sum/kg. The second pack is 1 000 sum/kg cheaper.
Class activity
“Which is cheaper?”: children get cards with two shop offers (for example 2 kg – 24 000 and 3 kg – 33 000), find the price of one kilogram and choose the cheaper one.
Practice
1
12 notebooks at 5 000 sum each. If the price becomes 3 times higher, what will the cost of 12 notebooks be in sum?
180000
2
7 kg of sweets at 4 000 sum/kg. If 10 times more, that is 70 kg, is bought, what will the cost be in sum?
280000
3
2 kg of apples cost 24 000 sum and 3 kg cost 33 000 sum. How many sum cheaper is 1 kg in the second pack?
1000
4
One watermelon and 4 melons cost 84 000 sum. The watermelon costs 20 000 sum. How many sum does one melon cost?
16000
5
A box holds 12 pens and costs 30 000 sum. How many sum are paid for 5 such boxes?
150000