Rosina, Ng, and Oscar can write a computer program in 3 days. Rosina can write the program in 8 days and Ng can do it in 10 days. How many days will it take Oscar to write the program? 68. Wiring. Janet can wire a house in 28 hr. Linus can wire a house in 34 hr. How long will it take Janet and Linus, working together, to wire two houses? 69. Roofing. Working alone, Russ can reshingle a roof in 12 hr. When Joan works with Russ, the job takes 6 hr. How long would it take Joan, working alone, to reshingle the roof? 70. Grading. Alma can grade a batch of placement exams in 3 hr. Kevin can grade a batch in 4 hr. If they work together to grade a batch of exams, what percentage of the exams will have been graded by Alma? 71. Home maintenance. Fuel used in lawn mowers and chain saws is made by pouring a 3.2-oz bottle of 2-cycle oil into 160 oz of gasoline. Gus accidentally poured 5.6 oz of 2-cycle oil into 200 oz of gasoline. How much more oil or gasoline should he add in order for the fuel to have the proper ratio of oil to gasoline?