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.
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