On test day, a teacher sharpens 19 pencils to lend out to his students. if n represents the number of pencils that the teacher lends out, which expression represents the number of pencils he has left?
a. b.
c. d.

19 - n = the amount of pencil's he has left

^ The amount of pencils he has, n, the amount of pencil he gives away
The answer is, 19 - n = the expression for the pencil's left

When rates are expressed as a quantity of 1, such as 2 feet per second or 5 miles per hour, they are called unit rates. if you have a multiple-unit rate such as 120 students for every 3 buses, and want to find the single-unit rate, write a ratio equal to the multiple-unit rate with 1 as the second term.

We need more info then that