The # of meals decreased by a total of 120 - 52 = 68 meals. Now in order to find the percent decrease, you must find what total percent 68 is out of 120.
68/120 = x/100, this ratio will give you the percent that 68 is out of 120 if you solve for x.
Using inverse operations you get:
6800 = 120x
x = 56.7% when rounded. Meaning that the total percent decrease was about 56.7
