Answer :

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

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