Answer
See below.
Work Step by Step
Let $x$ be the third grade.
a) Then $\frac{86+88+78+x}{4}\geq90\\86+88+78+x\geq360\\x\geq108$
Thus $x=100$ is not enough
b) Then $\frac{86+88+78+x}{4}\lt80\\86+88+78+x\lt320\\x\lt68$
Thinking Mathematically (6th Edition)
by Blitzer, Robert F.
Published by
Pearson
ISBN 10:
0321867327
ISBN 13:
978-0-32186-732-2
Chapter 6 - Algebra: Equations and Inequalities - 6.4 Linear Inequalities in One Variable - Exercise Set 6.4 - Page 385: 88
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