Q.The odds against $A$ solving a certain problem are 4 to 3 and the odds in favor of solving the same problem are 7 to 5, find the probability that the problem will be solved.

Odds against A solving, $4:3$, give $P(A\text{ solves})=3/7$. The problem states odds 'in favor of solving... are 7 to 5' without naming a second solver explicitly in the printed text, but this exactly matches the two-person 'John and Rafi' setup of Exercise 9.5 Q2 - so it is read the same way, as a second person (call them B) with odds in favour $7:5$, giving $P(B\text{ solve …

Convert both odds to probabilities and use the complement of 'neither solves', reading the question (whose printed wording omits naming the second solver) …

Treating this as a single-person problem and only using $P(A)=3/7$ as the final answer, missing that the 'odds in favor of so …