Question

The average height of 8 boys is 1 m 8 cm. The average height of 7 of them is 1 m 7 cm. How tall is the eighth boy in centimetres?

Answer 1

The average height of 8 boys is 1 m 8 cm. The average height of 7 of them is 1 m 7 cm. How tall is the eighth boy in centimetres?
Solution :
  Note : 1m = 100 cm
           Average height of 8 Boys =  1m 8cm = 108cm
             sum of heights of 8 boys = 8*108cm = 864cm
         Average height of 7 boys = 1m 7cm = 107cm
         Sum of heights of 7 boys = 7*107cm = 749cm

Let the height of 8th boy = x cm
           therefore ,
                   sum of heights of 7 boys + x = sum of  heights of 8 boys
                               749 + x = 864
                           or x = 864 - 749
                               x= 115cm

Hence , Height of 8th Guy = 115 cm
                                           = 1 m 15 cm

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Answer 2

let total of height 8 boys be x
let total of height 7 boys be y
so given x/8 = 1.08 m
        x = 8.64 m
and again y/7 =1.07 m
              y = 7.49 m
so height of 8 th boy = x - y = 8.64 - 7. 49 = 1.15 m 

1 m 15 cm
ANSWER