Question 847207
A/3 = B  
B + 40 = C    ->   A/3  + 40 = C
C * 2 = D    (which is less than 100) ->  (A/3 + 40)*2 = D (which is less than 100)

2/3 A + 80 < 100

2/3 * A < 20

A < 30

Assuming that A,B,C,and D are all integers, A can be any multiple of 3 less than  27. Even negative integers theoretically work. There is not a unique solution but any number following that form will work. If being an integer is not a requirement (you may want to check), then the only requisite is that A<30.  Even 29.9999 works.

Tests theory:

Say A = 6.

A=6
B=2
C=42
D=84<100

Say A = -15

A = -15
B = -5
C = 35
D = 70 < 100

etc.