Billy was counting the number of wheels and bike seats at the playground. he saw tricycles and bicycles. he counted 79 wheels and 28 seats. how many bicycles did billy see?

Let t = # of tricycles; let b = # of bikes.  

# of trike wheels is 3t; # of bike wheels is 2b.  Total number of wheels is 79, which equals 3t + 2b.   t+b = 28, so t=28-b.  

Subst. 28-b for t in the first equation:

3(28-b) + 2b = 79.  Then 84-3b + 2b = 79, or 5 = b (answer)

There were (28-5) = 23 trikes, 5 bikes, and 28 seats.