Excel - IF Function Problem - Expert Solution

Would very much appreciate help in creating a formula to calculate cost based on quantities that fall within set increments. Here's the scenario. There is one row for a customer to enter the desired quantity. Column A=quantity, Column B=per unit/increment cost of $8 (for each increment of 10 exceeding 20, as the first 20 are "free"), Column C=calculated total cost. If the customer buys any quantity within 1-20, cost is 0. If customer buys any quantity within 21-30, cost=8. If customer buys any quantity within 31-40, cost=16. And so on. EXAMPLE: If a quantity value from 1-20 is entered in Column A, the cost in Column C would be zero. For example, whether the quantity is 1 or 20 or anywhere in between, the cost would be $0 - the first 20 are free. If a quantity value from 21-30 is entered in Column A, the cost in Column C would be $8. For example, whether the quantity is 21 or 30 or anywhere in between, the cost would be $8 (first 20 free, any value that falls within the next increment of 10 [21-30], would be $8 total.) If a quantity value from 31-40 is entered in Column A, the cost in Column C would be $16. For example, whether the value is 31 or 40 or anywhere in between, the cost would be $16 (first 20 free, + first increment of 10 exceeding 20 [21-30] = $8, plus additional quantity falling within the second increment of 10 [31-40]=$8, for a total cost of $16.) And so on.