The cost of running an appliance varies jointly as the number of watts used, the hours of operation, and the cost per kilo-watt hour. A 6000 watt convection oven operates for 10 minutes for 15 cents at a cost of 7.5 cents per kilo-watt hour. what is the cost of cooking two meals if each meal takes 70 minutes to make?

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