Mark borrows $10$ dollars from Emily with a simple interest rate of $15\%$ everyday. What is the least integer number of days after which Mark will have to pay her back at least twice as much as he borrowed?
The simple interest for 1 day would be:
$10 x [15/100] =$1.50
$10 / $1.50 =6.66667days =~7 days in which Mark will have her back at least $20.
[7 x $1.50] + $10 =$20.50 This is what Mark will pay Emily back in 7 days.