Question

Your math tests scores are 68 78 90 and 91. What is the lowest grade you can get on the next test to still have an average at least 85

Answer 1

Let x be your fifth grade. The average will be

$\dfrac{68+78+90+91+x}{5}=\dfrac{327+x}{5}$

We want this average to be at least 85, so we have

$\dfrac{327+x}{5}\geq 85$

Multiply both sides by 5:

$327+x \geq 425$

Subtract 327 from both sides:

$x \geq 98$

So, you need at least 98.