The sum of two numbers is 21. The first number is 2/5 of the second number. What are the numbers?

Let's call the bigger number n

So the smaller number is 2/5n

So n + 2/5n = 21

Which is equivalent of n + 2n/5 = 21

Then you need to get rid of the fraction by multiplying everything by 5

5n + 2n = 105

Or simply

7n = 105

Divide both sides by 7 to isolate n

n = 15

So the larger number is 15

Now we need to work out 2/5 of 15 to work out the smaller number

(15/5 [the denominator]) * 2 [the numerator] = 6

So the two numbers are 15 and 6