How much water can be heated from 20 ° C to 60 ° C, if half of the energy obtained as a result

How much water can be heated from 20 ° C to 60 ° C, if half of the energy obtained as a result of burning 40g is suitable for heating it. coal?

Given:
t1 = 20 C
t2 = 60 C
m2 = 40 g = 0.04 kg – mass of coal
с = 420 J / kg * С – specific heat of water
q = 27 * 10 ^ 6 J / kg – specific heat of combustion of coal
Q1 = Q2 / 2
m1 -?
Q1 = c * m1 * (t2 – t1) – the amount of heat required to heat water
Q2 = q * m2 – the amount of heat released during the combustion of coal
c * m1 * (t2-t1) = q * m2 / 2
m1 = q * m2 / 2 * c * (t2-t1)
m1 = 27 * 10 ^ 6 * 0.04 / 2 * 4200 * (60-20)
m1 = 1080000/336000 = 3.2 kg