What is 4/10 of 81?

Often, in mathematics or everyday life, we need to find a fraction of a given number. For instance, how would you quickly calculate 4/10 of 81? We'll show you a straightforward method to achieve that.

How to calculate 4/10 of 81?

  1. Multiply the whole number by the numerator of the fraction: 81 × 4 = 324
  2. Divide this result by denominator of the fraction: 324/10 = 32.4
  3. Simplify by dividing both the numerator and denominator by their GCD (2):
    • 324/2 = 162
    • 10/2 = 5
  4. Which gives: 162/5

Since 162/5 is an improper fraction, the result could also be expressed as a mixed number: 32 2/5

How does it work?

The principle behind finding a fraction of a number, such as 4/10 of 81, is rooted in the method of cross-multiplication.

Consider the following proportion: 4 10 = x 81 (where x represents 4/10 of 81)

  1. Using the cross-multiplication method, multiply both numerators by their opposite denominators: 4 × 81 = 10 x
  2. Which gives: 324 = 10 x
  3. Divide both sides by 10 to isolate x: 324 10 = 10 x 10
  4. Which gives the result: 324 10 = x (or 162 5 = x )

Thus, 4/10 of 81 is equal to 162/5 (or 32.4 in its decimal form).