Normally I'd recommend the English File series from OUP. The Original "English File" is less challenging than "New Engish File". Both are still available.
But 20 hours? Hmm - most coursebooks have enough to cover 80 hours or more. Students may dislike paying for a whole book and using only part of it.
I can't vouch for any of these as I've never used them, but to get some titles of short course for further consideration, click this link, then click "select a category" then click "short courses":
Others will no doubt disagree, but I have little positive to say about Side By Side.