let P = Paul's rate, which will be jobs/hour
let M = Mary's rate, which will be jobs/hour
So P+M = 1/2 jobs/hour
P = 5/15 = 1/3 jobs/hour
So M = 1/2 - 1/3 = 1/6 jobs/hour
So in 12 hours, Mary can do 2 jobs. (If you want to write explicitly, multiply 12 hours by 1/6 jobs/hour).