I lost the little piece of paper that had my locker combination written on it! It takes me 10 seconds to try each of the combinations I can think of, and I have tried a total of 15 combinations so far! Looks like I am going to be late for class ...
Define a unit for the length of time spent trying combinations. Then enter a variable for the number of additional combinations tried and use this variable to write an expression for the length of time spent trying combinations.