9. A preschool playground has both bicycles and tricycles. There is a total of 30 seats and 70 wheels. How many bicycles are there? How many tricycles are there?

Step 1 ：Let the number of bicycles be x and the number of tricycles be y. We have two equations:

Step 2 ：$x+y=30$

Step 3 ：$2x+3y=70$

Step 4 ：Solve the system of linear equations to find the values of x and y.

Step 5 ：From the first equation, we get $y=30-x$

Step 6 ：Substitute this into the second equation: $2x+3(30-x)=70$

Step 7 ：Simplify the equation: $2x+90-3x=70$

Step 8 ：Combine like terms: $-x=-20$

Step 9 ：Solve for x: $x=20$

Step 10 ：Substitute the value of x back into the equation for y: $y=30-20$

Step 11 ：Solve for y: $y=10$

Step 12 ：$\boxed{{\displaystyle \text{There are 20 bicycles and 10 tricycles in the preschool playground.}}}$