122
Solving cost problems
Textbook: Part 4, lesson 122, pp. 37–39
GoalThe cost formula C = p × n, p = C : n, n = C : p and solving problems with a quantity–cost table.
New words
formula · formulatable · jadvalunknown · noma’lumdiagram · sxema
Explanation
The cost is usually written C, the price p and the quantity n. The formula is C = p × n, which gives p = C : n and n = C : p. If two of the three quantities are known, we find the third. When the price stays the same, the cost grows as many times as the quantity grows – a table is a handy way to show this.
Worked examples
The price is 8 000 sum/kg. Table: 1 kg – 8 000; 2 kg – 16 000; 3 kg – 24 000; 4 kg – 32 000 sum. Each time the cost grows by 8 000.
One pencil costs 1 800 sum and a box of pencils costs 36 000 sum. The box holds n = 36 000 : 1 800 = 20 pencils.
Class activity
“Formula triangle”: three groups write C, p, n on the board. The teacher covers one letter and the children say which operation finds it from the other two (multiply or divide).
Practice
1
The price is 9 000 sum/piece and the quantity is 7 pieces. What is the cost C in sum?
63000
2
6 m of cloth cost 72 000 sum. What is the price in sum/m?
12000
3
The price is 6 000 sum/kg and the cost is 48 000 sum. How many kg were bought?
8
4
4 identical cotton shirts cost 88 000 sum. How many sum does one shirt cost?
22000