Form 3 Physics 急!

Cindy is preparing milk for her baby. She mixes milk powder with 0.17 hot water at 61。C in a milk bottle. She finds the milk too hot and put the bottle in a water bath of 20。C to cool down the milk. The temperature of milk drop to 35。C after 5 minutes.
(the specific heat capacity of milk and water are 4200Jkg-1)
energy lose from milk to the water (water bath) is 18564J.
What is the mass of water in the water bath?
E=mc delta T
18564=(m)(4200)(35-20) <-點解會係35-20?
m =0.295kg