What must sara get on her next test to average 80% if her previous test scores were 78, 88, 83, and 81?

70

Explain:

80*5 = 400
this tells us that the total scores before
dividing must be 400

400 - (78+88+83+81) = 70
this tells us that we lack +70

in conclusion, to get an average of 80% on Sara's test scores; She must receive a grading of 70% in her next test.

Aand b are correctly balanced reactions. they have the same number of atoms in both sides of the reaction