Question

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?

Answer 1

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%