this is a porportion, she is walking 3.9 every hour so that is 3.9/1 (1 representing the one hour) and she only walked .72 hours and we dont know far she walked for, so its x/.72 we cross multiply (not cross cancel) and we get 3.9×.72=x
we multiply those decimals and we get 2.808 miles she had walked. Hope that helps.
