Question

Mariana averaged 80 on her first three math exams. The first score was 86. The second score was 74. What was the third exam score?​

Answer 1

The three scores are 86, 74, and an unknown score x. The average (i.e. the sum divided by how many there are) is 80, so we have

$\dfrac{86+74+x}{3}=80$

Multiply both sides by 3 and you get

$160+x=240$

Subtract 160 from both sides to get

$x=240-160=80$

Note: you could solve this exercise very quickly if you notice that the two numbers, 86 and 74, are symmetrical with respect to 80, which is the average. So, the third number must be the average.