Find the area of a triangle with legs that are: 15mm,10mm,20mm

To calculate the area of triangle with three sides we use Heron's formula, 
Area=sqrt(s(s-a)(s-b)(s-c))
where:
s=(a+b+c)/2
thus, 
s=(15+10+20)/2
=22.5
hence the area will be:
Area=sqrt(22.5(22.5-15)(22.5-10)(22.5-20)) 
Area=sqrt(22.5*7.5*12.5*2.5)
=sqrt5273.4375
=72.62 mm^2