Give the step-by-step reasoning process and then the final answer. At the park, Dimitri saw families riding bicycles and tricycles. Bicycles have two wheels and tricycles have three wheels. 6 adults were riding bicycles and 15 children were riding tricycles.  How many wheels did Dimitri see at the park?

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Dimitri saw 2 x 6 = 12 wheels on the bicycles. He saw 15 x 3 = 45 wheels on the tricycles. He saw a total of 12 + 45 = 57 wheels.
The final answer: 57.