Three energy saving improvements are advertised to save 25%, 55% and 20% of the energy used. a homeowner makes these three improvements in succession. what overall percentage saving can be expected?

Let the original electricity units used without energy savings be 100 units. 
After the first energy saving improvement, the energy used 
is 100 U - 25% of 100 U = 75U

After the second energy saving improvement, the energy used is
75U - 55% of 75 U = 33.75 U

After the third energy saving improvement, the energy used is
33.75 U - 20% of 33.75 U = 27U

Therefore, the overall percentage saving is 100U - 27U = 73U or 73%