In 21 hours, a heating plant consumes 7 liters of fuel. How much does it consume in 90 hours?
Let x be the consumption in 90 hours.
We have: 21/7 = 90/x because the heating plant always consumes fuel at the same rate
Multiplying both sides by 7x, we get 21x = 90 * 7
This can be simplified to 21x = 630
So, the consumption in 90 hours is x = 630 / 21 = <<630/21=30>>30 liters.
#### 30