Five donuts and 3 cinnamon rolls cost $11.50. eight donuts and 6 cinnamon rolls
cost $20.50. how much does a donut cost? how much does a cinnamon roll cost?

The cost of one donuts is $1.25 and the cost of one cinnamon is $1.75

Step-by-step explanation:

Let

x-----> the cost of one donuts

y ----> the cost of one cinnamon

we know that

-----> equation A

-----> equation B

Solve the system by graphing

Remember that the solution of the system of equations is the intersection point both graphs

using a graphing tool

The solution is the point (1.25,1.75)

see the attached figure

therefore

The cost of one donuts is $1.25 and the cost of one cinnamon is $1.75

I'd like to offer the alternative perspective. I think that learning grammar rules through a written account of the system (text books) is something that is only useful to a language learner under the following conditions:
they have already learned how to have a conversation in the language 
they are naturally curious now about how the patterns in how it fits together 
they would like to learn how to read in the language
they would like to improve their accuracy for a specific purpose (eg professional or educational)