The cost of going to the big playland park for a family is given by c = 15x + 50, where c represents the cost and x represents the number of family members. how many family members went if it cost them $125?

As you have already given the equation and what each variable represents, I'll restate it here;
C = 15x + 50; C represents the total cost, and X represents the number of family members.
The total cost was $125, so we should substitute C with 125.

125 = 15x + 50; now, all we have to do is solve for x with basic algebra.
First, let's isolate the variable, so subtract 50 from both sides of the equation.

15x = 75; divide both sides by 15 to solve for x, the number of family members that went to the park.
15x / 15 = x
75 / 15 =5

We are left with the following solution:
x = 5

We can conclude that 5 family members went to the park.

I hope this helps!