Answer:
B. 180
Explanation:
There is a condition that each chest needs to be at least 20 pounds and we can use that to our advantage. Because we want one of the boxes to be really heavy, we need the other three to be really light. Let's set the heavy box as variable y. Set each of the other three boxes equal to 20. Because the combined weight is 240, then the equation so far is 240 = (20*3)+y.
Now, we can solve for y. 240-60 = 180, so the heaviest box has to be 180 lb.