Silvia’s test scores are 86, 78, and 92. what must she score on her fourth test in order to have an average of 85 for all four tests?

a.) 74

b.) 90

c.) 75

d.) 84

The answer is D.
To find this, you need to do (86+78+92+x)/4=85
You then multiply 4 to each side (to isolate x)
You are left with (86+78+92+x)=340
You can then subtract 256 (the value of 86+78+92) from both sides.
You then have x=84
Therefore, you know she must score an 84 on her next test to average an 85.