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 *between*15 and 70 percent. If your
calculation wasn't go back and try again.

The real answer for those who want to check their calculations.