Grade 4 · Unit 1: Number and place value

Mental subtraction

Find differences quickly.

Mountain heights

Saka Haphong, among the highest hills in Bangladesh, is about 1050 m high. Keokradong is about 986 m high. What is the difference?

Counting up

Count up from 986 to 1000: 14. 1000 to 1050: 50. Difference = 64 m.

Constant difference

Moving both numbers by the same amount keeps the gap. 5003 − 1998 is the same as 5005 − 2000. Now it is easy: 3005.

Subtracting multiples

8400 is 84 hundreds. 2700 is 27 hundreds. 84 − 27 = 57. So the answer is 5700.

What I learnt

• I can count up to subtract.
• I can use constant difference.
• I can subtract multiples of 100 and 1000.