20% of 700 = 140
So you have 140 Kw/h at 22.4p and 560 Kw/h at 9.8p
or (140*22.4)+(560*9.8)=8624p or £ 86.24
or do you want:
a problem to solve:
If for the first 20% of the time a kw/h costs 22.4p, then after that it only costs 9.8p per kw/h
What is the average price of 1 kw/h? and how do you solve this?
edit: this is to be worked out assuming the total kw/h used is 700.