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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 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

More from Mental maths:

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