0.4a+70%*0.4*(84-a)=30.72

(0.4-0.28)a=30.72-23.52

a=60

the second question

Set the power consumption x+60 degrees.

60*0.4+x*0.4*70%=(X+60)*0.36

24-2 1.6=(0.36-0.28)X

x=2.4/0.08

x=30

Electricity consumption in September is 30+60=90 degrees.

Pay 90*0.36=32.4 yuan