couple of word equations that I need to get down by thursday

I really need to understand how to do these equations. similar questions were on the pre-test but these are from the book. I think if I can understand these I will understand the test coming up.

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After giving a math test, Yolanda can grade 20 tests in 60 minutes and shawn can do it in 68 min. How long will it take them to grade the tests if they work together?

** Major one I have problems understanding on setup

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During an afternoon practice, two football team members took the same amount of time to run sprints of 40 yards and 50 yards, respectively. If one of the football team members ran 2 yards per second faster than the other, find the rate of each team member.

D=r*t I know that 40/r and 50/r+2

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*The distance a car can travel varies directly with the amount of gas it carries. On the highway, a car travels 80 miles using 4 gallons of fuel. How many gallons are required to travel 200 miles?

***The one that really kills me

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I=P/R

A constant voltage is applied to a circuit that has a variable resistor. Varying the resistance causes the current to vary inversely. At a resistance of 8 OHMS, the current is measured to be 12 AMperes. Find the current when the resistance is 20 OHMS.

(Headbang)(Headbang)

I really want to pass this test.