We know that in 2 ½ hours (2.5 hours), we can create 3 cakes hence:
C = k t
where C is number of cakes, k is constant, and t is time
k = 3 cakes / (2.5 hours)
k = 1.2
C = 1.2 t
So for t = 7 ½ hours = 7.5 hours
C = 1.2 (7.5)
C = 9 cakes in 7 ½ hours