Markup is the difference between the scarf's selling price and it's original cost.
The scarf was originally $12, let
S1 = 12
The cost is $15, let
S2 = 15
Markup, M = S2 - S1
= 15 - 12
= 3
$3$
Markup Percent is now:
Markup divide by original cost, times one hundred
That is;
3/12 * 100
= 100/4
= 25 percent