In general for this kind of problem, a line representing the ages would help you. But I hate drawing, even a line, so I'll try to explain it with words
Let BertNow be the age of Bert now XD. Let BillNow and BenNow be the age of Bill and Ben now (do you follow for the moment ? lol). I call them this way in order not to confuse with a,b,c when reading the problem.
All the green sentences will refer to the "condition" : something happened when blablabla
I think the red was is actually an "is"... referring to the age of Bill now. Otherwise, this piece of information can't be used further.When Bert was just one year younger than Bill was when Ben was half as old as Bill will be 3 years from now
Suppose that the green sentence happened X years ago.
Transcription of the green sentence gives : BenNow-X=(BillNow+3)/2 ---> X=BenNow-(BillNow+3)/2
Transcription of the black sentence gives : BertNow-X=BillNow-1
Substitution gives : BertNow-BenNow+BillNow/2+3/2=BillNow-1 ---> BertNow-BenNow-BillNow/2+5/2=0
Suppose it was Y years ago.Ben was twice as old as Bill was when Ben was 1/3 as old as Bert was 3 years ago
The green sentence gives : BenNow-Y=(BertNow-3)/3 ---> Y=BenNow-(BertNow-3)/3
And the black sentence : BenNow-Y=2*(BillNow-Y) --> BenNow+Y=2*BillNow
Substitution : BenNow+BenNow-(BertNow-3)/3=2*BillNow ---> 2*BenNow-BertNow/3-2*BillNow+1=0
Suppose it was Z years ago.when Bill was twice as old as Bert, Ben was 1/4 as old as Bill was one year ago.
Green sentence : (BillNow-Z)=2*(BertNow-Z) ---> Z=2*BertNow-BillNow
Black sentence : BenNow-Z=(BillNow-1)/4
Substitution : BenNow-2*BertNow+BillNow=BillNow/4-1/4 ---> BenNow-2*BertNow+3*BillNow/4+1/4=0
Let a=BertNow, b=BenNow, c=BillNow.
Thus we have the system :
I hope it's clear !