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Victoria jogs 12 miles to the park along a flat trail and then returns by jogging on a 20-mile hilly trail. She jogs 1 mile per hour slower on the hilly trail than on the flat trail, and her return trip takes her two hours longer. Find her rate of jogging on the flat trail.

One gardener can mow a golf course in 4 hours, while another gardener can mow the same golf course in 6 hours. How long would it take if the two gardeners worked together to mow the golf course?

In the following exercises, add.

$\frac{7x^2}{x^2-9}+\frac{21x}{x^2-9}$