Find the number of bicycles and tricycles Help for my son.  My math is a bit rusty and I'm trying to remember how to go about answering this question:  "There are 3 times as many bicycles in the playground as there are tricycles.  There is a total of 81 wheels.  What is the total number of bicycles and tricycles in the playground?"     

Hint:  Let there be $b$ bikes and $t$ trikes.  Each sentence provides an equation, giving two simultaneous equations in two unknowns.  Or group three bikes with a trike (based on the first sentence).  How many wheels does it have?  How many groups are there?
