15% and the corresponding 0.15 is by necessity unit-less; it's just a multiplier. If you run up a $1 bill and wish to tip 15%, multiply $1 by 0.15, obtaining $0.15.
Then the total due would be $1 plus $0.15, or $1.15.
Note that $0.15
-------- = 0.15 is unit-less
$1
