ANS -50
As the speed of esclator is constant therefore no. of steps covered by esclator during travelling of either of P or Q remains same the only difffrence is that it would get subtracted in case of P(think as a case of downstream) and get added in case of Q (as case of upstream) and therefore we can conclude the no. of steps covered by esclator would lie in mid i.e (70-30)/2=20 .hence total no. of steps would be (30+20 or 70 -20) =50