# The mean of 35 ob

We know that Sum of observations = Mean ×Total number of observations

Sum of 35 observations = 20×35=700

Sum of first 18 observations = 15×18=270

Sum of last 18 observations = 25× 18=450

While adding first 18 observations and last 18 observations, we add 18th observation twice so,

18th observation= Sum of first 18 observations + Sum of last 18 observations - Sum of 35 observations

18th observation= 270 + 450 – 700 =20

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