Division/Multiplication by 25

It is easy to remember that 100/25 = 4, and therefore 1000/25 = 40, 10000/25 = 400, etc.

For example, what is 4725 / 25? This is 4×40 + 7×4 + 1 = 160 + 28 + 1 = 189.

Equivalenetly, sometimes it is easier to multiply by 4 than to divide by 25. For example, divide 4732 by 25. We already know that 4725/25 = 189. What remains is to divide 7 by 25. Multiply 7 by 4 and divide by 100 to obtain 0.28 = 7/25. The final result is then 4732/25 = 189.28.

Assume now we wish to find 48×25:

48×25 = 48×100/4 = (48/4)×100 = 1200.

This trick is useful even when the division by 4 is less straightforward:

63×25 = 63×100/4 = 6300/4 = 3150/2 = 1575.

As you see, dividing by 4 can be done in two steps.


Related material
Read more...

  • Multiplication by 9, 99, 999, (Multiply + Subtract) etc.
  • Squaring 2-Digit Numbers
  • Division by 5
  • Multiplication by 2
  • Multiplication by 5
  • Multiplication by 9, 99, 999, etc. (Something Special)
  • Product of 10a + b and 10a + c where b + c = 10
  • Product of numbers close to 100
  • Product of two one-digit numbers greater than 5
  • Product of 2-digit numbers
  • Squaring Numbers in Range 26-50
  • Squaring Numbers in Range 51-100
  • Squares of Numbers That End in 5
  • Squares Can Be Computed Squentially
  • How to Compute Fast Any Square
  • Adding a Long List of Numbers

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