The ratio P : Q is in proportion with 3 : 7. If the expression 13(P − Q)2(P + Q)PQ(2P + Q) − 7 is simplified and written in the fractional form mn, where m and n are coprime integers, find the value of n − m.

If all the girls in school B and school C participated in the function, find the ratio between the number of boys in school B and school C who participated in the function.

Which of the following examples represents an inverse proportional relationship (reverse movement)?