The average of 4 numbers is 98. If 3 of the numbers are 86, 87, and 91, what is the 4th number?

Average of 4 numbers, if the numbers are x, y, z, t then the average is (x+y+z+t) / 4=average so

average=98

if 3 of the numbers are 86, 87 and 91 then what is the fourth number

assign

x=86

y=87

z=unknown

91=t

(86+87+z+91) / 4=98

do parenthasees first

(264+z) / 4=98

multiply both sides by 4 to get rid of fraciton

264+z=392

subtract 264 from both sides

z=128

the fourth number=128