Explanation

Let's denote the unknown number as x.

Given that 3/4 of 28 is equal to 21/5 of x:

(3/4) * 28 = (21/5) * x

First, calculate 3/4 of 28:

(3/4) * 28 = 21

Now, equate this to 21/5 of x:

21 = (21/5) * x

Multiply both sides by 5:

105 = 21x

Divide both sides by 21:

x = 105 / 21

x = 5