Originally Posted by

**SpringFan25** it is not correct to divide 6,300,000 by x and multiply 68400 by x. Depending on how you approach the question, you should do one or the other.

6,300,000 is the production cost of the cup, and it is constant. it doesn't decrease because you washed the cup 1 more time.

I dont think you have written the question correctly either

Do you mean

how many times must I use a re-usable cup for **the average energy cost** to match the energy cost for 1 use of a disposable cup?

in this case, the average cost per use of the disposable cup is

$\displaystyle \frac{6,300,000}{x} + \frac{64800x}{x}$

$\displaystyle = \frac{6,300,000}{x} +64800$

not $\displaystyle \frac{6,300,000}{x} + 64800x$