Question 673146
let x = the grade Nancy must make on the fourth exam to have an average of at least 80

to get an average you must add the exam grades and divide them by the amount of grades

72+83+91+x/4>=80
246+x/4>=80
multiply both sides by 4 to get rid of it
246+x>=320
subtract 246 from both sides
x>=74

she must receive at least a 74 on her fourth exam