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Write as you read
A + B + C = 100 (1)
A + B = 35 (2)
B + C = 75. (3)
From equation (1), subtract equation (2). You will get C = 100-35 = 65.
From equation (1), subtract equation (3). You will get A = 100-75 = 25.
Now from equation (1)
B = 100 - A - C = 100 - 25 - 65 = 10.
ANSWER. 25 potatoes in bin A; 10 potatoes in bin B; 65 potatoes in bin C.
Solved.
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This problem is very special.
It has a special setup and a special method of solution.
Such problems arise from time to time in different guises.
MEMORIZE how to setup such problems and how to solve them.