One hint is that to consider some of the differences between this and the high case. In the high case there were two sources of information which suggested that higher than 50 percent chance (the 85 percent baserate and the 70 percent clinical information), so we know the answer for that case should be higher than either 85 and 70 since each contributes to our confidence. For the low case it is different. The 70 percent individuating information adds to our confidence while the 15 percent base rate should subtract from it. In combination that means that the correct answer should be between15 and 70 percent. If your calculation wasn't go back and try again.
The real answer for those who want to check their calculations.