Find 4 numbers in AP whose sum is 20 and the sum of the whose squares is 120

Let the 4 numbers be variables.

For example, let the first number be x.

The second number would be x+d
The next would by x+2d
And the next, x+3d

Their sum is 20...
x + (x+d) + (x+2d) + (x+3d) = 20

The sum of their squares is 120...
x^2 + (x+d)^2 + (x+2d)^2 + (x+3d)^2 = 120

This should be easy enough.

If you take the terms in A.P as x-3d, x-d ,x+d and x=3d, you can easily go through the solving step.

the last term is x+3d