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