first, translate all the words into mathematical equations. so that will be:
2a=b
3b=c
d=3c

the next step is to substitute all the equations we've got. C is equal to 3b (c=3b), and we know that b = 2a. So we can write c= 3b= 3(2a)= 6a.
in other words, we state all the variable in terms of one variable (in my case, I chose variable a cause it's easy)
it will be like this:
b= 2a
c= 3b= 6a
d= 3c= 18a

the final step is to write the quantities (A and B) in terms of variable a and do the math. you'll see that quantity A and B are equal: both final result is 1/2

hope this helps