exam Maths Puzzle Numerical Ability

3 over y square minus 4 + 1 over 4 over y - 2 + 1

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exam Other Question

I dont understand how you are supposed to justify each step because it just doesnt make sense to me... Could someone help me please? The problem is:
A ball is thrown upward from the roof of a building 100m tall with an initial velocity of 20m/s. When will the ball reach a height of 80m?