If

C(x) = 1.5x / (100 -x)

then

what is x when C(x) = 5?

That is what the problem ask you.

5 = 1.5x / (100 -x)

Clear the fraction, multiply both sides by (100 -x),

5(100 -x) = 1.5x

500 -5x = 1.5x

500 = 1.5x +5x

500 = 6.5x

x = 500/6.5 = 76.923 percent

That is 77 percent of the population participating. ---answer.