Quantitative aptitude

Question

1. P can alone do a task in 15days and Q can do the task in 20 days. If they work through for 4 days. How much work is left incompletly.

Answer 1

Each day P does 1/15th of the work

Each day Q does 1/24th of the work

Working together, each day they do 1/15 +1/24 of the work, that is 39/360 or 13/120 of the job per day

Working four days they do 52/120 or 13/30 of the job, so that 17/30 of the job is left over.

Answer 2

1 - (4/15 + 4/20) = ?