How many joules are needed to change the temperature of 100. g of water from 20 to 45

This can be found by using Q=mcΔt, where Q is the heat in joules, m is mass in grams, c is the specific heat, and Δt is the change in temperature (final-initial)

So,

Q = (100. g) (4.18J/g°C) (15°C)

Q = 6897J needed