Before the clowns arrived only 35% of the students were happy . if 117 were not happy how many children wee there in all

First we subtract 100 - 35 to get 65. (Because 35% is out of 100%)
Since we already know there are 117 students, we can automatically assume 65%= 117
117 is 65% of what? 
We use this formula to find out: y/p% = x
Plug in the numbers so it's now 117/65%= x
Or to make it easier, 65% is also 0.65 (you can find by dividing 65/100)
So now we have 117/0.65= x
Work that through to get x= 180
Hope that helps!