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Which expression demonstrates the use of the commutative property of addition in the first step of simplifying the expression (-1+i)+(21+5i)+0

Tyler went to the supermarket to buy food for a food pantry. he has $36, and can carry up to 20 pounds of food in his backpack. pasta costs $1 for a 1-pound package. pasta sauce costs $3 for a 1.5 pound jar. let x = the number of packages of pasta and y = the number of jars of pasta sauce. one package of pasta is the right amount to go with one jar of pasta sauce. what is the best numbers of packages of pasta and jars of pasta sauce to buy for the food pantry? how many packages of pasta? how many jars of pasta sauce? explain your reasoning.

Ireally don't understand this, i'm really an idiot but i want to know how to figure this out. !