Laura earns $66 washing 6 cars. At this rate, how many cars did Laura wash to earn $88?

Laura washed 8 cars to earn $ 88

Step-by-step explanation:

Number of cars : earnings = 6 : 66

6 : 66 :: x : 88

Product of mean  = Product of extremes

 66 * x = 6 *88

        x =

       x = 8

The answer is: The mistake was in the second expression.

The second expression should be the follow:

The students are working with a cosine identity, cosine sum.

According to the theorem, cosine of a sum will be:

Where, for this case:

So, substituting we have:

So, the mistake was in the second step expression.

The expression should have been:

But why the result was correct even using a wrong expression?

The answer to that question is based on the value of sin(π) which is equal to 0.

Have a nice day!