Hi Mareiya R.
Since you are given the total amount, the amount used for the loaf of bread and left overs, the difference between these values divided by the ounces per muffin which is also given can be used to find the number of muffins made
50 oz = Total
21.02 oz = Left over
16.2 oz = Used for the loaf of bread
0.71 oz = is needed for each muffin
Let n = number of muffins
0.71n = 50 - (16.2 + 21.02)
0.71n = 12.78
n = 12.78/.71
n = 18
You can check this value as follows
50 -(16.2 + 12.78) = 21.02
50 - 28.98 = 21.02
21.02 = 21.02
I hope this answers your question.