I'm assuming that since 87% failed, that would the remaining 13% was a success. If that assumption is correct, than we simply have to take:
.13 * 13531 = 1759.03 products were successful.
The names PNM and MPN are perfectly valid. The order of the letters doesn't matter. In fact, there are 6 ways to name this plane if we just use the 3 letters. If we include "plane Z", then there are 7 total ways to name the plane.
For some reason, your teacher wants P to be the third letter as s/he mentioned MNP and NMP. But again, the order doesn't matter.