Assuming that there is no heat loss, and the cup is perfectly insulated, all of the energy lost by the lead will be absorbed by the water.
The heat absorbed is given by:
Energy = mass * specific heat capacity * change in temperature
The specific heat capacity of water is 4.18 joules per gram.
In order to compute the mass, we will use the fact that the density of water is one gram per milliliter. Therefore, the mass of water is 50 grams.
Now,
Q = 50 * 4.18 * (20 - 18)
Q = 418 J
418 Joules of energy were released by the lead.
