Riddle: three men get a hotel room which is 30$. they each pay $10. the manager comes back later and realizes he overcharged them, and should have only asked $25 for the room. so he takes $5 out of the register, puts $2 in his pocket, and gives the men $1 each. so now the men have paid $9 each instead of 10. BUT 9+9+9=27 and 27+2(the money the manager put in his pocket)=29. so where is the other dollar?
Answer: there are two ways to answer this riddle, but both have the same answer "there was never another dollar":
the first is algebraic:
(10x3)-3+2=29 because 10x3=30-3=27+2=29
whereas (10x3)-(3+2)=30 because 10x3=30 3+2=5 and 30-5=25
the other is simpler math:
there was only $30 in the register after the transaction. the clerk took out $5, gave 3 to the men and kept two for himself, but he still took out only 5. and $30-5=25 dollars that are in the register.