How many 7 pound sacks does he have?

Question

Eric has 8 sacks of flour. Some of his sacks weigh 3 pounds, and the others weigh 7 pounds. The total weight of his flour is 36 pounds. How many 7 pound sacks does he have?

Answer 1

f+F=8

3f+7F=36

f+F=8 -----> f=8-F

substitute

3(8-F)+7F=36

24-3F+7F=36

24+4F=36

4F=36-24

4F=12

F=3 sacks of 7-pound flour

f=5 sacks of 3-pound flour