If your school allows guess n check method, u may want to use that to try..

For example 40 notebooks and staplers in total, notebook cost $5 & staplers cost $3. Overall expense $166.

So lets say 20 notebooks n 20 staplers are bought.. then 20x5 (notebook) + 20x3 (staplers) = $160
Since we are $6 short , we will increase the count of the notebook purchase
instead of the stapler count purchase..

otherwise, if u increase the count of the stapler purchase, the overall expense would be lesser than $160 & that does not meet the requirements and the scenario given in the question. You may want to try to understand it better.. getting back to your question,

21x5 (notebook) + 19x3 (stapler) = $162
22×5 + 18×3 =$164
23x5 + 17x3 = $166

Thus we know that 23 notebooks and 17 staplers have been purchased to ensure $166 has been spent on the 40 notebooks and staplers..

This is guess and error method. Also known as trial and error. Provide workings to ensure your teacher knows you know what you are doing:)

Any queries, u can reply to ask!