The sum of three numbers is twenty. the second number is four times the first and the sum of the first and third is eight. what are the three numbers?

Let's call the three numbers x, y, and z. 

The numbers are 3, 12, and 5.
x + y + z = 20
y = 4x
x + z = 8
We can rewrite the first equation because y is 4x, so 
x + 4x + z = 20
5x + z = 20
And now we can use simultaneous equation rules to solve

5x + z = 20
x + z = 8

5x + z = 20
5x + 5z = 40
(subtract)
-4z + -20
÷ -4
z = 5

x + z = 8
x + 5 = 8
x = 3

y = 4x
y = 4 × 3
y = 12

I hope this helps!