The number of elephants at the circus was two fifths the number of horses. If there were 16 elephants, how many horses would there
be?

So first off, lets start off with what we know:

The total sum of the angles in a triangle is always 180 degrees.

Angle A = 60 degrees. (They told us so in the picture!)

That means that:

Angle B + C = (x + 20) + 3x

And B + C = 180 - 60 = 120 degrees.

Therefore (x + 20) + 3x = 120 degrees.

So it follows that if we simplify we get:

4x + 20 = 120 degrees

4x + 20 - 20 = 120 - 20

4x = 100 degrees

x = 25 degrees.

Then we know that:

B = 3x = 3 * 25 = 75 degrees.

And:

C = x + 20 = 25 + 20 = 45 degrees.

We can check by doing:

95 + 75 + 60 = 180 degrees!

So we have solved the problem.