In a row of 45 boys; P is 18th from the left end and Q is three positions to the right of P. What is the position of Q from right end?

25^th

As P is 18^th from the left end; therefore position of Q is 21^st from left end.
Required answer;
Total number of person  = Left position  + Right position – 1
                                 45 = 21 + Right position  – 1
                right position = 25