Lisa's grades on four exams are 90, 92,86 and 78. what is the lowest grade she can receive on the next exam to average an 85?

Hey mate!
You stuck?
Let's figure this out!

(80+92+86+78+x)/5 > 85
(336+x)/5 > 85
336+x > 425

x > 89 is the grade she needs to get an average of 85. If the requirement is that it must be greater than 85, then:
89+1= 90 is the lowest she needs to have an average greater than 85

Hope this helps!

The answer is -1,19,39,59