You go out to pick blueberries and black berries. you know that to make a black and blue pie, you need to have 5 blueberries for every 2 blackberries you pick. if an entire pie has 140 berries, how many blueberries does it contain and how many blackberries does it contain?

5 blueberries for every 2 blackberries 
black=x blue=y

140= 2x + 5y
    -5y           -5y

-5y + 140= 2x (divide everything by 2 to get x by itself

-2.5y +70= x (plug in to find Y)

140= -2.5 + 70 + 5y
-70            -70

70= 2.5y (divide by 2.5)

28=Y

plug in to find xagain XD

140= 2x + 28
-28         -28

112= 2x (divide by 2)

56=x

28 blueberries and 56 blackberries

i accidentally made it longer than i intended to, i am pretty sure there is a better way to do this, but i hope i helped you somehow