Yes. Correct.

Since the same number of cars are washed, this means that there must be more big cars washed by Team D than Team A.

We can think of it as replacing a small car to be washed, with a big car. In a sense, we are 'changing' the Team A to Team D.

For each small car replaced, the Team earns $5 more.

$15 - $10 = $5

Number of small cars to be replaced
= $30 ÷ $5
= 6

This means that there are actually 6 more big cars washed by Team D than Team A.

Number of big cars Team D washed
= 5 + 6
= 11


There is no need to use assumption

Thanks, but the question is how do i show the calculation to account for 5 big cars? Because is 5 + 6 (6 cars from previous step)

You don't need to show any working for the 5 big cars.

It is understood that to get the number of big cars for Team D, you're using the difference of 6 big cars and adding it to the 5 big cars for Team A to get this

Ok got it thanks