Kinda got it on 7th grade but never given the answer but now since I read it again I understand it easier
Initially, total room cost for 3 families are $300, which means each family pay $ 100. After that return $50. (Bell boy return $10 to each family and kept himself $20.) That's mean the family only fork out $90 ($100 - $10). $90 x 3 = $270 + $20 (bellboy kept it) = $290. Amazing $10 missing... Haha.....
