Unit Rate
a)
If 3 tacos cost $18, the unit rate is $18 / 3 = $6.
This is the unit price of one taco, i.e., each taco cost $6.
With that information, we can calculate the cost of any given number of tacos.
For example,
4 tacos cost 4 * $6 = $24.
5 tacos cost 5 * $6 = $30
6 tacos cost 6 * $6 = $36
b) If you buy t tacos for c dollars, the unit rate is calculated as above, dividing c by t, that is:
unit rate (cost per taco) = c/t