The first computer does 1/8 of the work in one day and the second does 1/5 of the work in one day. So both together do 1/8 + 1/5 of the work in one day. You need to simplify 1/8 + 1/5 into a fraction of the form A/B: then if both computers do A/B of the work in one day, they do all the work in B/A days.

In my day people were digging holes and the like. See the The Human Element in Mathematics by Stephen Leacock.