Holly has 20 balls, and there are 100 balls in total.
So there are 100-20 = 80 balls left.
Alex had 2 * the amount Holly had, which is 20.
So Alex has 20 * 2 = 40 balls.
Together, they have 60 balls.
Which means 100-60 = 40. So Ben has 40 balls.
Check:
Holly: 20
Alex: 40
Ben: 40
Add them: 20 + 40 + 40 = 100
A rectangle has four sides, two equal to the width and two equal to the length. So, the perimeter is

If we know that the width is 5, and the perimeter is 125, we have

In your case, the unkown is called x, but the concept is the same:

where x is the length of the rectangle.
The answer is B.
When you set up a ratio of all the corresponding sides, the ratios all equal 2:1