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A student in an algebra class has test grades of 81 and 92. What must he score on the third test in order to have an average of 90?

Let $$x$$ be his grade on the third test.

$$\frac{81+92+x}{3} = 90$$

$$81 +92 +x = 270$$

$$173 + x = 270$$

$$x= 270-173$$

$$x = 97$$ Answer