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This diagram shows the formula: Total monthly cost in dollars equals 25 plus, then open parenthesis, the total number of minutes used minus 30, close parenthesis, multiplied by 0.2. A bubble points to the 25 and says “This is the monthly charge of $25.” A bubble points to the bracket and says “The expression in the brackets works out the extra minutes that have been used.” A bubble points to the 0.2 and says “This is the cost of each extra minute in $. 20¢ = $0.20.”

 9.3.3 Math in Business