Rate of X=1/15 Rate of Y=1/18 Pick a magic number and take it as a dummy work amount. magic number=LCM of 15 and 18=i.e. 90 => Rate of X=6/hr => Rate of Y= 5/hr work done by x alone in 6 hrs=6x6=36 work done by both x and y in 3 hrs=(6+5)x3=33 work left= 90-(36+33)=21 time taken by y to comple...