area of triangle = 1/2 base height
what we know:
area of triangle = 120
height = 15
now we just substitute it in:
120 = 1/2 base 15
dividing by something is the same as multiplying by its reciprocal, so:
120 2 = base 15
240 / 15 = base
16 = base
so, your base is 16 inches.