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To re-phrase this: 50 is a certain percent of 90.
this can be written mathematically as 50=x%*90
let's divide both sides by 10:
5=x%*9
and divide both sides by 9:

which in decimals is approximately 0.56 (but not exactly).
so 0.56=x%
and the x then 56
so 50 is
approximately 56% of 90:
exactly 56%of 90 is 50.4
Answer:
about $2.54 per pound of sugar
The percent of water would be 40%