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This problem trains for: SAT-I, AMC-8.

Trurl and Klapaucius have to total 450 hours of training in a simulator. One of them has trained 20% more than planned and the other one has trained 10% more hours than planned. The total number of hours has thus become 515. How many more hours has the one trained than the other?

(characters from Stanislaw Lem's "Cyberiad")

Let t be the time planned for Trurl's training. Let k be the time planned for Klapaucius' training. Then, in hours:

In fact, they trained:

After simple manipulations:

Therefore, one of them trains 35 hours more than the other.