EdExcel A-Level Maths October 2020 Paper 3 (Mechanics)

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for question 5 I tried doing it another way but it did not work, could you tell me where I am going wrong.

work out time for horizontal to reach 50m (100/square root 2 U).
then substituite that T value in V= U + at with v=0 (as at greatest height the vertical velocity=0) U= Usin45. a= -9.8

I don't understand why I get a different value of 31 instead of 28