Answer:
17/50
Explanation:
When the bus is on time 4/5 of the time, it is late 1/5 of the time.
Mason arrives at 9 am when the bus is on time 3/4 of the time, so he doesn't arrive at 9 am 1/4 of the time.
Mason arrives at 9 am when the bus is late 3/10 of the time, so he doesn't arrive at 9 am 7/10 of the time.
The probability for Mason not to arrive at 9 am when the bus is on time is:
4/5 × 1/4 = 1/5
The probability for Mason not to arrive at 9 am when the bus is late is:
1/5 × 7/10 = 7/50
The probability for Mason not to arrive at work by 9 am is:
1/5 + 7/50 = 17/50.