Question

D is partly constant and partly varies with V. When V = 40, D = 150, and when V = 54, D = 192. Find D when V = 73.

Answer 1

D = mV + C , ( compare this to your familiar y = mx + b)
when V=40, D=150
150 = 40m + C **
when V=54, D=192
192 = 54m + C ***
subtract ** from ***
42 = 14m
m = 3
sub into ** , 150 = 120 + C
C = 30

so you have D = 3V + 30

sub in your given value of V