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 ：\(\boxed{\text{There are 20 bicycles and 10 tricycles in the preschool playground.}}\)