Her grandmother pledged to donate $1 per mile she runs.
Her mother pledged $14 flat (no matter how many miles she runs she will donate $14)
We want both her grandmother and her mother to pay the same amount of money. We know that each donor will owe $14 because of her mothers flat fee (her fee will not change). To find this answer, we need to find out how many miles ($1) it takes to reach $14. Here its simple, it would take 14 miles to get her grandmother to owe $14, the same amount as her mother.
Janets mother and grandmother will each owe $14 if she walks 14 miles