Help! M Math question.

Ben bought 5 books.

Given that:
The most expensive book cost $25
The cheapest book cost $17

Also, since the median is $18, that meant at least 2 books cost $18.

So, we knew the cost of 4 books already.

Let x be the cost of the 5th book.
Since teh mean price of the books is $20,
therefore　(25 + 17 + 18 + 18 + x) ÷ 5 = 20
　　　　　　　　　　　　　78 + x = 100
　　　　　　　　　　　　　　　x = 22

The price of the 5 books : $17, $25, $18, $18, $22

Let X be the price of the other books.
(X+18+18+25+17)/5=20
X+78=100
X=22

The answer is $22.