Well this is a simple trig question, for this use the sine rule:
Whatever you are looking for will be on the top so in this case length will be on the top.
a/sinA = b/sinB, a = 15, b=b, A = 60 and B = 90 (Assumption)
Re-arrange for b:
b = a(sinB)/SinA = 15(sin60)/sin90
b = 13ft
v = s/t where v = velocity, s = distance and t = time taken
we are looking for t so:
t = s/v = 13/75
t = 0.173
So the time in minutes would be 0.173 x 60 = 10.4 minutes
