15. To travel a fixed distance, the number of gallons needed is inversely proportional to the miles per gallon rating P of the car that gets 42 miles per gallon (mpg) needs 33 gallons to travel the distance.

a. Find an equation of variation
N=
b. A car that gets 77 mpg needs _____ gallons of gas.


13. The number of instructions N performed per second by a computer varies directly as the speed S of the computer's internal processor. A processor with a speed of 20 megahertz can perforn 1,800,000 instructions per second.
a. Find an equation of variation
N=
b. The same processor perform _____ instructions if it is running at a speed 180 megahertz.

a. gallons needed=K/P

to find K, K=33*42 from the data.
then N=22*42/P

b. N=cS find C from the data.