math 1
posted by MNHS .
The average of 10 numbers is 12. If one number is removed, the average is 11. What number is removed?

( a1 + a2 + a3 + a4 + a5 + a6 + a7 + a8 + a9 + a10 ) / 10 = 12 Multiply both sides by 10
a1 + a2 + a3 + a4 + a5 + a6 + a7 + a8 + a9 + a10 = 120
( a1 + a2 + a3 + a4 + a5 + a6 + a7 + a8 + a9 ) / 9 = 11 Multiply both sides by 9
a1 + a2 + a3 + a4 + a5 + a6 + a7 + a8 + a9 = 99
( a1 + a2 + a3 + a4 + a5 + a6 + a7 + a8 + a9 ) + a10 = 120
99 + a10 = 120 Subtract 99 to both sides
99 + a10  99 = 120 99
a10 = 21
Number 21 is removed. 
the total of the 10 number is 120
solve
(120x)/9 = 11
120x = 99
x = 21
x = 21
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