Interesting Maths Quiz

Last friday after bible study, I and my friends talk about an interesting quiz..Just wanna share this quiz below..Sometimes we will discuss maths questions which cant be solve while we gave tuition and the advice can be quite rewarding cos abt half of our group took double mathematics in JC...

Here are the quiz:
Three travellers went to a hotel overnight.They paid $30 dollars(each paid 10 dollars) but the Landlord told them it is $25 dollars as there is a promotion discount..She gave them 1 dollar each and kept 2 dollars as tips(altogether 5 dollars)..Hence each traveller paid 9 dollars instead..However, 9x3 =27+2 dollars tips..Where is the 1 dollar gone? Think abt it..