There are 30 tricycles and cars parked outside a shopping mall.There are a total of 105 wheels altogether.How many tricycles are there?

Let's solve this problem step by step:

Step 1: Let's denote the number of tricycles as T and the number of cars as C. We know from the problem that T + C = 30.

Step 2: We also know that each tricycle has 3 wheels and each car has 4 wheels. So, the total number of wheels is 3T + 4C = 105.

Step 3: Now we have a system of two equations, and we can solve it. Let's express C from the first equation: C = 30 - T.

Step 4: Now let's substitute C in the second equation: 3T + 4(30 - T) = 105.

Step 5: Simplify this equation: 3T + 120 - 4T = 105.

Step 6: Combine like terms: -T + 120 = 105.

Step 7: Solve for T: T = 120 - 105 = 15.

So, there are 15 tricycles.