By foot: 1% of 7=0.07 are late;
By bike: 2% of 8=0.16 are late;
By bus: 5% of 10=0.50 are late.
So, in any given period, 0.73 students are late. For example, in 100 days (2500 "trials" in about 5 school months) 73 students are late. So the probability of lateness is 73/2500=0.0292, or 2.92%.
Continuing with the example, of the 73 students who came late, 16 came by bike, so 16/73=0.22 or 22% (approx) is the probability.