After turning off C, rate of A + B = ( \frac{1}{6} + \frac{1}{4} = \frac{4}{24} + \frac{6}{24} = \frac{10}{24} = \frac{5}{12} ) tank/hour.
Remaining empty: ( 1 - \frac{7}{12} = \frac{5}{12} ) tank. 2017 psle math questions and answers
Time needed = ( \frac{\text{remaining}}{\text{rate}} = \frac{\frac{5}{12}}{\frac{5}{12}} = 1 ) hour. After turning off C, rate of A +
If total ribbon = 8.4 m, 0.8 m per small, 1.2 m per large, large = small + 3: ( 0.8S + 1.2(S+3) = 8.4 ) → ( 2S + 3.6 = 8.4 ) → ( 2S = 4.8 ) → ( S = 2.4 ) (still not integer). After turning off C