3/2 + 21 = 22.5 or 45/2, but why? what are the steps to get that answer?

Its worth 10000000 points need asap if you answer correctly ill mark brainliest

The u-drive rent-a-truck company plans to spend $13 million on 320 new vehicles. each commercial van will cost $25 comma 000, each small truck $50 comma 000, and each large truck  $80 comma 000. past experience shows that they need twice as many vans as small trucks. how many of each type of vehicle can they buy?

Ineed to know the answer to this algebra question.