A student scored 78 and 95 on his first two quizzes. Use a compound inequality to find the possible values for a third quiz score that would give him an average between 80 and 90, inclusive.

Question

anonymous · Answer

67 <= x <= 97
78 <= x <= 95
80 <= x <= 90
86 <= x <= 87

anonymous · Answer

Multiply the desired average by 3 for the 3 tests then subtract the two known values.

anonymous · Answer

I'm sorry I'm really lost.

anonymous · Answer

To get an average of 80: $80 \times 3 = 240$ So you need a total of 240 points.
$240 - 78=162; 162-95=67$ Therefore, you would need a 67 on the 3rd test to get the lower average of 80.