The first thing we must do for this case is to find the total weight of the watermelons.
We have then:

We now look for the amount of watermelons that people ate.
We have then:

Therefore, the amount of watermelon remaining is:

We note that the amount of watermelon remaining is equal to 8.5 pounds.
Answer:
the amount of leftover watermelon is: equal to 8.5 pounds