The statement "P is partly constant and varies partly as Q" means P consists of a constant term plus a term directly proportional to Q. Thus, the relationship is:

P = a + bQ, where a is the constant part and b is the constant of proportionality.

Given:
P = 32 when Q = 16 → a + 16b = 32
P = 20 when Q = 12 → a + 12b = 20

Subtract the second equation from the first:
(a + 16b) - (a + 12b) = 32 - 20
4b = 12
b = 3

Substitute b = 3 into the second equation:
a + 12(3) = 20
a + 36 = 20
a = -16

Thus, P = -16 + 3Q

When Q = 28: P = -16 + 3(28) = -16 + 84 = 68