Suppose that r varies directly with s and inversely with t, and r=2 when s=3 and t=12. what is the value of r when s=5 and t=4?

i already know the answer but i'd like to know how to find it.

Tom is the deli manager at a grocery store. he needs to schedule employee to staff the deli department for no more that 260 person-hours per week. tom has one part-time employee who works 20 person-hours per week. each full-time employee works 40 person-hours per week. write and inequality to determine n, the number of full-time employees tom may schedule, so that his employees work on more than 260 person-hours per week. graph the solution set to this inequality.