Let a and b be the two numbers.

since, the sum of the two numbers is 87, then a + b = 9

now, since one number is twice another number, we can say...

a = 2b

Now we can solve the equation. Substitute the value 2b into a,

2b + b = 9 The equation looks easy now since variables are the same, we can add them.

2b + b = 9

3b = 9

b = 9 / 3

b = 3

so one number is 3, solve for the other number...

since he sum of the two numbers is 9,

a + b = 9, we have the value of the other number b, so...we can write...

a + b = 9

a + 3 = 9 then..

a = 9 - 3

a = 6

Therefore, the two numbers are 3 and 6