Answer:
$67 profit
Explanation:
The average of three numbers is their sum, divided by 3. Let x represent the profit during the third month. Then the average monthly profit is ...
(-29 + 115 + x)/3 = 51
86 +x = 153 . . . . . multiply by 3, collect terms
x = 67 . . . . . . . . . . subtract 86
Their profit the third month was $67.