Question 663839
Let TestScore4 be the test score on the test that will give Emily an average of 70 across all four test
Average is given by the TestSumTotal/NumberOfTestsTaken

{{{Average = (TestScore1 + TestScore2 + TestScore3 + TestScore4)/4}}}
{{{70 = (69 + 71 + 67 + TestScore4)/4}}}
{{{280 = (69 + 71 + 67 + TestScore4)}}}
{{{280 - 69 - 71 - 67 = TestScore4}}}
{{{73 = TestScore4}}}

Make sense?