ok this one is really confusing me..i dont get how to solve the equation!!

A bookstore sold 18,000 paperbacks one month. This was 10% less than the number of paperbacks the store sold the previous month. The following equation represents this situation, where x represents the number of paperbacks sold the previous month.

x − 0.1x = 18,000

How many paperbacks did the store sell in both months combined?

Question

ok this one is really confusing me..i dont get how to solve the equation!! 

A bookstore sold 18,000 paperbacks one month. This was 10% less than the number of paperbacks the store sold the previous month. The following equation represents this situation, where x represents the number of paperbacks sold the previous month.

x − 0.1x = 18,000

How many paperbacks did the store sell in both months combined?

anonymous · Answer

dumb equation
if the book store sold 18,000 books and it is 10% less than the previous amount, it is 90% of the previous amount
lets call the previous amount P 
then $18,000=0.9P$ so 
$$P=18,000\div .9$$

anonymous · Answer

why divided by .9?

anonymous · Answer

because wait...ohh i think i got it...:)