Let's use the following notation:
p1 - probability of getting delayed on M1
p2 - probability of getting delayed on M2
p3 - probability of getting delayed altogether
p1 = 50%
p2 = 75%
I will assume that probability of taking each road is the same. If it isn't, this answer is incorrect!
p3 = 50% * p1 + 50% * p2
p3 = 0.5 * p1 + 0.5 * p2
p3 = 0.5 * 50% + 0.5 * 75%
p3 = 0.5 * 0.5 + 0.5 * 0.75
p3 = 0.25 + 0.375
p3 = 0.625
p4 - probability of NOT being delayed
p4 = 1 - p3
p4 = 1 - 0.625 = 0.375 = 37.5%
Hope this helps
