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

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