Answer:
<em>Farmer Bill sold 120 lb of potatoes</em>
Step-by-step explanation:
Suppose farmer Bill had x lb of potatoes. He sold a quarter to the first person. A quarter is written as (1/4)x. After that, Bill had x-(1/4)x = (3/4)x.
Then he sold 4/9 of what was left to the second person, thus it was left 1-4/9=5/9 of the remaining potatoes. Multiplying both factors:

This is the remaining portion of the potatoes.
Finally, he sold 4/5 of it and the last 10 lb. This means 1 - 4/5 = 1/5 of the remaining is equivalent to 10 lb:

Thus:

x = 120 lb
Farmer Bill sold 120 lb of potatoes