Three messenger services deliver to a small town in Oregon. Service A has 60% of all the scheduled deliveries, service B has 30%, and service C has the remaining 10%. Their on-time rates are 80%, 60%, and 40% respectively.

a. What proportion of the packages in the town are delivered on time?

e. What proportion of the packages in the town are delivered late?

Selected answer: a. 70% e. 30%

It seems i could just average the on time delivery rates to get the first answer. But that would be .6 and that's clearly the wrong answer.

To calculate, I could use a proportion formula.

a : b :: c : d ---> a/b=c/d