Simple math.

We she has a total off 100 colored counters

55 are reds

the rest are green or black

green are 4x than black

we can write it as,

100 = 55 + 4x + x

x will represent black while 4x will be black times 4 (or simply green).

Now time to compute.

move 55 to the left and change sign.

100 - 55 = 4x + x

Now add 4x and x and compute 100 - 55

we have,

45 = 5x

divide both 45 and 5x to 5.

(45 = 5x)/5

we have,

9 = x

therefore Sarah has a total of 9 black counters.

But your question is how many green counters she has?

So we need to multiply black counters to 4 to get it.

so we have,

9 * 4 = 36

so Sarah has a total of 36 green counters.

To check

we need to add all red, green and black counters. It should total to 100 colored counters.

55 + 36 + 9 = 100

55 + 45 = 100

100 ✔ 100

I hope it helps.