Each Monday at 5pm,
1. he drops off 3 dirty shirts that he wore F, Sa, & Su.
2. he picks up 4 clean shirts that he will wear Tu, W, Th, & F.

You might ask, "Where is the shirt for Monday?" Answer: He's wearing it!

Each Friday at 5pm,
1. he drops off 4 dirty shirts that he wore M, Tu, W, & Th.
2. he picks up 3 clean shirts that he will wear Sa, Su, & M.

You might ask, "Where is the shirt for Friday?" Answer: He's wearing it!

One might think that 7 shirts would be enough for 7 days.  However, think
about the shirt he is wearing while he is dropping off and picking up.
That shirt is going to be dirty until the next drop-off-pick-up day. 
So the shirt he is wearing while he is dropping off and picking up --
is never part of the 7 shirts total he is picking up and dropping off. 
So he needs 1 additional shirt, for there are 8 shirts present at 5pm
every Monday and Friday, counting the ones he is dropping off, the ones
he's picking up, and the one he is wearing.

Answer: He needs 8 shirts.

Edwin