The attack does 10 base damage, plus 20 for each heads.

So, if you flipped two out of three heads, you have the 10 guaranteed damage, 20 damage, and 20 more damage.

10+20+20 = 50

For two heads, this attack does 50 damage.

For one head, it'd only be one 20, and for three heads; three 20s. the 10 at the beginning is the base damage of the attack, meaning if you get NO heads (You unlucky mess!), the attack will still do 10 damage.