Question

If grapes are 92% water and raisins are 20% water by weight, then how much did a quantity of raisins, which currently weighs 10 pounds, weigh when all the raisins were grapes

Answer 1

10 lbs of raisins contains 20% water by weight, so they contain

0.20 × (10 lbs) = 2 lbs

of water. This means the non-water portion of the raisins weighs 8 lbs. This weight stays the same throughout the drying process.

As grapes, they had a total weight of

8 lbs + w

where w is the water weight, and w happens to be 92% of this total weight. So

w = 0.92 × (8 + w)

Solve for w :

w = 7.36 + 0.92w

0.08w = 7.36

w = 92

Then the total weight of the grapes before they dried out is 100 lbs.