Average Of 8 Numbers Is 20, That Of The First Two Is 15.5 And That Of The Next Three Is 211/3, The 6th Is Less Than The 7th By 4 And 7 Less Than The 8th. What Is The Last Number?

Sol:

Sum of 8 numbers = 20 × 8 = 160

Sum of first 5 numbers = \( 15.5 * 2 + \frac{64}{3} * 3 \)

= 31 + 64 = 95

Sum of first 5 numbers = (160 – 95) = 65

Let the 6th, 7th and 8th numbers be x, y and z respectively.

y − x = 4—-[1]

z − x = 7—-[2]

z − y = 3—-[3]

From equation, [1] and [2], we get:

x + y + z = 65 —-[4]

From equation, [3] and [4], we get:

z – x = 7

= z = 25

Therefore, the last number is 25

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