Write the fraction 36/40 in simplest form.

36/40 is the same as 9/10.  
36 goes into four, nine times. 36 / 4 = 9.
40 goes in four, ten times. 40 / 4 = 10.
If you are dividing a number by a fraction, lets say the numerator, then the same number you divided has to be divided by the denominator.
For example:
5/30
The numerator 5 goes into five, one time, because 5 / 5 = 1
You divide the same number 5, for the denominator also. 
30 goes into five, six times, because 30 / 5 = 6
The simplest form of 5/30 is 1/6.
Hope this helps!
-AphmauSenpai-