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.

# Answer

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<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
In 21 hours, a heating plant consumes 7 liters of fuel. How much does it consume in 90 hours?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

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