how to get a cell to have a data value depending on what is selected from a drop down

ok ladies and gentlemen total novice here.

im trying to speed up my mileage document, i only travel to a few different offices and so would like to be able to choose the place i visited from a drop down menu and then the milage to that place to automatically appear in the next cell.

i have attached a what i have been playing with.

any help on this would be much appriciated.

many thanks