AlgebraMediumExtending a constant filling rate
Algebra practice question
A tank is being filled at a constant rate. At noon it holds 5 litres, and at 4 p.m. it holds 17 litres. How many litres will it hold at 10 p.m.?
Answer: 35
The rate is (17 − 5)/4 = 3 litres per hour. From noon to 10 p.m. is 10 hours: 5 + 3 × 10 = 35 litres.
Common wrong answers
- If you answered 23
- 23 is 17 + 6, adding only two more hours' worth after 4 p.m.; 10 p.m. is six hours later, so add 18.
- If you answered 30
- 30 is 3 × 10 without the 5 litres already in the tank at noon.
- If you answered 50
- 50 is 5 × 10, using the starting amount as the rate.
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