2. Rounding numbers

Rounding numbers to the nearest 1, 10 or 100 can help with lots of mental maths problems. Here are some examples:

• To add 9 to another number, add 10 and then subtract 1
  36 + 9 = 36 + 10 - 1 = 45
• To add 18 to another number, add 20 and then subtract 2
  48 + 18 = 48 + 20 - 2 = 66
• To add 97 to another number, add 100 and then subtract 3
  439 + 97 = 439 + 100 - 3 = 536
• To add 0.9 to another number, add 1 and then subtract 0.1
  3.7 + 0.9 = 3.7 + 1 - 0.1 = 4.6
• To subtract 8 from another number, subtract 10 and then add 2
  46 - 8 = 46 - 10 + 2 = 38
• To subtract 19 from another number, subtract 20 and then add 1
  63 - 19 = 63 - 20 + 1 = 44
• To subtract 0.9 from another number, subtract 1 and then add 0.1
  8.2 - 0.9 = 8.2 - 1 + 0.1 = 7.3