Take
    R = event that a customer is "risky credit;" P(R) = 0.25
    R' = event that a customer is not "risky;" P(R') = 0.25
    OD = event that a customer's account is overdue; P(OD|R) = 0.5; P(OD|R') = 0.1.

First Step
Rephrase the question as "Calculate the probability that ____ given ____ ."

You are asked to compute the percentage of overdue accounts held by "risky credit" customers. In other words, given that an account is overdue, what is the probability that it is held by a "risky credit" customer? Therefore, we can rephrase the question as:

"Calculate the probability that a customer is a "risky credit" customer given that the customer's account is overdue ."

In symbols, this is P(R|OD).

Bayes' theorem now says that

P(R|OD)	=	P(OD|R)P(R) P(OD|R)P(R) + P(OD|R')P(R')
	=	(0.5)(0.25) (0.5)(0.25) + (0.1)(0.75)	=	0.625	,

Therefore, 62.5% of overdue accounts are held by customers in the "risky credit" category.