There is other way to solve such problems. Printer x alone can do 8.33% (1/12%) work in an hr. Similary y and z can do 6.66% and 5.55% work in an hr respectively. Together y and z can do 12.22% of work in an hr. So % work done ratio for x / (y + z) = 8.33/12.22 so time raio = 12.22/8.33 approx 22/15...