4/7 of the students at a park were boys. if there were 66 girls, how many students were there at the park?

If 4/7 of the students were boys, that means 3/7 of the total were girls.
Let x be the total number of students, that means 3/7 of x = 66 girls.

In other words,
(3/7)x = 66.

Solve for x:

Get rid of the denominator, so multiply 7 on both sides. 
3x = 462

Divide 3 on both sides:
x = 154

So there were a total of 154 students at the park :)

We can double check this by taking 4/7 (the fraction of boys) of 154 and adding it to 66 (the total number of girls) and see if it adds to 154!

4/7 x 154 = 88

88 + 66 = 154

It worked!