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