Question

u = 75 , f = 25 , find v for convex lens

Answer 1

1/f= 1/v + 1/u
thus
1/25 = 1/v + 1/75
1/25 - 1/75 = 1/v
75-25/1875 = 1/v
50/1875 = 1/v
thus
1/v = 1/37.5
thus
v = 37.5

Answer 2

This is your answer