The speed of the current in a river is 6mph. a ferry operator who works that part of the river is looking to buy a new boat for her business. every day, her route takes her 22.5 miles each way against the current and back to her dock, and she needs to make the trip in a total of 9 hours. she has a boat in mind, but she can only test it on a lake where there is no current at all. how fast must the boat go on the lake in order for it to serve the ferry operator's needs?

Ariana starts with 100 milligrams of a radioactive substance. the amount of the substance decreases by 20% each week for a number of weeks, w. the expression 100(1−0.2)w finds the amount of radioactive substance remaining after w weeks. which statement about this expression is true? a) it is the difference between the initial amount and the percent decrease. b) it is the difference between the initial amount and the decay factor after w weeks. c) it is the initial amount raised to the decay factor after w weeks. d) it is the product of the initial amount and the decay factor after w weeks.