Some bikes and trikes are on the playground. there are 7 seats and 19 wheels. how many bikes are there? how many trikes are there?

Let x the number of bikes, and y the number of trikes. 
We have the equations:
x+y=7 (only one seat )
2x+3y=19 (two wheels for a bikes and three wheels for each trike)
Solving the above system for x and y we get:
x=2, y=5. 
So there are two bikes and 5 wheels. 
For solving, use x = 7-y, then using the second equation 
2(7-y)+3y=19. solving the above equation for y we get y=5 then 
deduce x=7-y=7-5=2.