I find the easiest way to solve these kinds of problems is to set up a table like this if you are unsure how to set up the equations and solve simultaneous equations.
=Now-------Then=
=G|D--------G|D=
=1|5----------9|13=
=3|7---------11|15=
=4|8---------12|16=
=5|9---------13|17=
=6|10--------14|18=
First column = answer choices
Second column = D is 4 years older than G
Third/Fourth = their ages after 8 years
So Greg current age is 5.