Suppose: C=cost c=cost per kilo-watt hour w=number of watts h=number of hours thus C=kwhc where: k=constant of proportionality: k=C/(whc) but C=15 when w=6000, h=10/60=1/6 hours, c=7.5 cents per kw/h thus k=15/(6000*1/6*7.5) k=0.002 thus: C=0.002(whc) hence: when h=70*2=140=140/6=70/3 hours the cost will be: C=0.002(6000*70/3*7.5) C=2100 cents C=$21
