Subtracting big numbers

In your book, Maths Mela (Chapter 4, We the Travellers — II), a minibus had 18 l of fuel, and after refuelling it had 65 l. To find how much was filled, we work out 65 − 18. In the ones, we cannot take 8 from 5, so we regroup: 1 ten becomes 10 ones, giving 15 ones. 15 − 8 = 7. Only 5 tens are left, and 5 − 1 = 4 tens. So 47 l was filled. Check: 18 + 47 = 65.

Big numbers are subtracted the same way. The sea route from Mumbai port to Chennai port is 2,700 km. A ship first reaches Cochin port after 1,083 km. How much further must it go? 2,700 − 1,083: the ones and tens are 0, so we regroup 1 hundred as 10 tens, and then 1 ten as 10 ones. Ones: 10 − 3 = 7. Tens: 9 − 8 = 1. Hundreds: 6 − 0 = 6. Thousands: 2 − 1 = 1. The ship has 1,617 km more to go. Check: 1,083 + 1,617 = 2,700.

The longest land route in the world, from Talon (Russia) to Sagres (Portugal), is 15,150 km. The longest highway in Africa, from Cairo to Cape Town, is 10,228 km. How much longer is the land route? 15,150 − 10,228 = 4,922 km. Check: 10,228 + 4,922 = 15,150.

Keep the digits lined up, ones under ones, and regroup whenever the top digit is smaller. Then always check by adding the answer to the number you took away.