Answer:
a) 7.14% probability that Benny was learning to ride a bike using the training wheels
b) 28% probability that Benny was learning to ride a bike using the training wheels
Step-by-step explanation:
Bayes Theorem:
Two events, A and B.
In which P(B|A) is the probability of B happening when A has happened and P(A|B) is the probability of A happening when B has happened.
Benny came home crying with a bruise on his knee, after falling. His mom is trying to guess how Benny was trying to learn to ride a bike.
a) Assuming that the probability that Benny was using each of these 3 methods is equal, what is the probability that Benny was learning to ride a bike using the training wheels?
So
Event A: Benny fell
Event B: Benny was using training wheels.
The probability that Benny was using each of these 3 methods is equal
This means that 
He did some research, and discovered that if he rides a bike with training wheels, the probability of falling is 0.1;
This means that 
Probability of falling:
1/3 of the time, he uses training wheels. With training wheels, the probability of falling is 0.1.
1/3 of the time, he uses the bike without training wheels. Without training wheels, the probability of falling is 0.5
1/3 of the time, he uses the unicycle, for which he has an 0.8 probability of falling. Then
So
7.14% probability that Benny was learning to ride a bike using the training wheels
b) Since Benny's mom knows Benny so well, she knows that the probability that he was using training wheels is 0.7, regular bike is 0.2, and unicycle is 0.1. What is the probability now that Benny fell while using the training wheels?
Similar as above, just some probabilities change.
Event A: Benny fell
Event B: Benny was using training wheels.
The probability that he was using training wheels is 0.7
This means that 
He did some research, and discovered that if he rides a bike with training wheels, the probability of falling is 0.1;
This means that
Probability of falling:
0.7 of the time, he uses training wheels. With training wheels, the probability of falling is 0.1.
0.2 of the time, he uses the bike without training wheels. Without training wheels, the probability of falling is 0.5
0.1 of the time, he uses the unicycle, for which he has an 0.8 probability of falling. Then
So
28% probability that Benny was learning to ride a bike using the training wheels
.