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 :)
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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!