3
Change by trading notes
Discover Mathematics P2: Term 2, Lessons 61–70
GoalSubtract amounts of money by trading a bigger note for ten smaller ones.
New words
change · the money you get back when you pay with a bigger noteregroup · trade one note for ten notes of the next smaller kinddifference · the answer to a subtraction
Explanation
To subtract money, start with the ones. If you do not have enough ones, trade one 10 LE note for ten 1 LE notes. In 52 − 27, take 5 tens and 2 ones. There are only 2 ones, so trade one ten for ten ones. Now there are 4 tens and 12 ones. 12 − 7 = 5 ones, and 4 − 2 = 2 tens. The answer is 25. If there are not enough tens, trade one 100 LE note for ten 10 LE notes. Change is the money you get back: pay with 50 LE for something that costs 38 LE, and the change is 50 − 38 = 12 LE.
Worked examples
52 − 27: there are not enough ones, so trade one ten for ten ones. 12 − 7 = 5 and 4 − 2 = 2. The answer is 25 LE.
140 − 65: split 65 into 60 and 5. First 140 − 60 = 80, then 80 − 5 = 75. The answer is 75 LE.
Youssef pays with a 50 LE note for a ball that costs 38 LE. His change is 50 − 38 = 12 LE: one 10 LE note and two 1 LE notes.
Class activity
Shop and change game. One player is the shopkeeper and one is the customer. The shopkeeper says a price under 100 LE, the customer pays with a 100 LE note, and the shopkeeper gives change using play notes. Check by adding the price and the change: you should get 100.
Practice
1
Subtract 61 − 38. Do you need to trade?
Yes: 1 one is less than 8, so trade a ten. 11 − 8 = 3 and 5 − 3 = 2. The answer is 23.
2
Mina has 200 LE. She buys shoes for 135 LE. How much is left?
200 − 135 = 65, so 65 LE.
3
Seif pays 100 LE for a toy that costs 72 LE. What is his change?
100 − 72 = 28, so 28 LE.
4
Check your answer to 52 − 27 = 25 by adding. What do you add?
25 + 27 = 52, so the answer is right.
5
Subtract 40 − 16.
There are 0 ones, so trade a ten: 10 − 6 = 4 and 3 − 1 = 2. The answer is 24.