Answer:
(a) 412 +A -B
(b) 412 +A -B + (A-20) -(B+14)
(c) 378 +2A -2B
Explanation:
Water flowing in adds to the amount in the reservoir. Water flowing out subtracts from the amount in the reservoir.
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(a) 412 + A - B . . . . at the end of January
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(b) Flowing in was (A -20), flowing out was (B +14). In February, we start with the amount at the end of January.
(412 +A -B) +(A-20) -(B +14)
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(c) Simplifying the above, we have ...
412 -20 -14 +2A -2B = 378 +2A -2B