The heat change by the each sample is proportional to its mass times the temp change. So if we use T for final temp, the energy lost by the 100g is:100(80-T)
The heat gained by the 60g is 60(T-20)
Set the two equal and do the algebra. T=60
(I should have multiplied each by the heat capacity of water, but when I set the two terms equal, it would just cancel out.)