I have a pouch with pens and pencils in it. At the moment, the ratio of pens to pencils is 3:5. If Phil gives me 3 pens in exchange for 9 of my pencils, I will have an equal number of pens and pencils. How many pens will that be?
P==Pens, c==pencils
P / c == 3/5.....................(1)
[P + 3] / [c - 9]==1.........(2), solve for P, c
P ==18 - original number of Pens
c ==30 - original number of pencils.
18 + 3 ==21 Pens
30 - 9 ==21 pencils
+83 
