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