Kari thinks, "x is the difference between 87 and 34.25 is this true or not?

This is true because it is an unknown number so it is replaced by a variable.

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.