The 2 parallel lines that surround the number are called "absolute value signs" everything inside them has an outcome of a positive number and in this case the number is 15. Like I said the number(s) inside the absolute value signs have to have a outcome of a positive number, notice in c and d there is a negative sign outside the absolute value signs. Therefore you multiply the negative seperately so, 15(-1) is -15.
Or, if your money is earning a 5 percent interest rate, you'll double it in 14.4 years (72 divided by 5 equals 14.4). If your money is earning a measly 1 percent interest rate, it will take you—yep, you guessed it—a whopping 72 years to double it.
