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Explanation:
x = original amount of caster oil (in quarts)
Since 15% remains, this means you used 85% of the original amount x.
Note how 15% + 85% = 100%
So,
85% of x = 34 quarts
0.85x = 34
x = 34/0.85
x = 40
You started with 40 quarts of oil.
15% of that remains, meaning 0.15*40 = 6 quarts of oil are left over.
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We could also solve like this
(amount used)/(amount total) = (percent)/(100)
34/x = 85/100
34*100 = x*85
3400 = 85x
85x = 3400
x = 3400/85
x = 40
So we end up with the same x value as before, and we follow the same set of steps at the end of the first section to end up with the answer of 6 quarts.
