book Maths Puzzle Numerical Ability

i asked this question and got an answer but i still do not understand it, can someone make it a little bit more easier to understand, if thats even possible ;) thank you very much.

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MULTIPLY AND SIMPLIFY

5i(4i-6)
A ski lift takes a skier up a slope at 50 meters/min.The skier then skis down the slope at 150meters/min. If one round trip takes 24 mins, how long is the slope?