One month Chris rented 7 movies and 9 video games for a total of $71. The next month he rented 5 movies and 3 video games for a total of $31. Find the rental cost for each movie and each video game.

Simultaneous equations:
7m+9v=$71
5m+3v=$31

multiply the whole of the second equation by three to get:
7m+9v=$71
15m+9v=$31
now subtract the second equation from the first to get: 8m=$40 so a movie costs $5

now substitute the movie back into:
7(5)+9v=$71
35+9v=$71
9v=$36
so a video game costs $4