Question

One fifth of a number is subtracted from four fifth of the same number. If the result is 9, what is the number. ​

Answer 1

Let x be the unknown number. One fifth of this number is x/5, one fourth of this number is x/4. So, we have

$\dfrac{x}{4}-\dfrac{x}{5} = \dfrac{5x-4x}{20} = \dfrac{x}{20}$

We know that this difference equals 9, so we have

$\dfrac{x}{20}=9$

Multiply both sides by 20 to get

$x=180$