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!
