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

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