Adding and subtracting are linked

In your book, Maths Mela (Chapter 4, We the Travellers — II), a minibus has 18 l of fuel left. After refuelling, the fuel meter shows 65 l. How much fuel was filled? We find 65 l − 18 l = 47 l. Is that right? We can check by adding: 18 + 47 = 65. Yes! Adding the answer back to the number we took away gives the number we started with.

Addition and subtraction are linked like this. One addition fact gives two subtraction facts. If 78 + 164 = 242, then 242 − 164 = 78 and 242 − 78 = 164. The total is 242, and taking away one part leaves the other part.

One subtraction fact also gives other facts. If 198 − 98 = 100, then 100 + 98 = 198, and 198 − 100 = 98. If 921 − 137 = 784, then 784 + 137 = 921 and 921 − 784 = 137. So when we already know one fact, we do not need to work the others out again.

Always check a subtraction by adding. The book asks for 82 − 37. The answer is 45, and the check is 37 + 45 = 82.