How much of a 90% alloy must be combined with 70% gold alloy in order to get 60 ounces of an 85% gold alloy?
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The 4 subintervals are given: [2, 4], [4, 7], [7, 9], and [9, 10].
Each subinterval has length: 4 - 2 = 2, 7 - 4 = 3, 9 - 7 = 2, and 10 - 9 = 1.
Over each subinterval, we take the value of the function at the right endpoint: 3, 8, 15, and 18.
Then the integral is approximately
so 78.0 is the correct answer.