In 21 hours, a heating plant consumes 7 liters of fuel. How much does it consume in 90 hours?
Stream of thoughts:
30
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 = 30 liters.

.

The final answer: 30