Using new vendors model A small retailer is considering various monthly plans for purchasing electricity. They can pre$-$purchase kWhs at the cost of $4\mathrm{.}5$ cents per kWh$.$ If they dont use all of the kWhs they purchase, then there is no refund for the unused electricity. If they need more kWhs than they purchase, the extra kWhs are purchased at the rate of $6$ cents per kWh$.$ What is the cost of over$-$estimation?What is the cost of under$-$estimation? Consider the small retailer example above. What is the critical ratio?