Question

The average length of 20 bricks is 360 cm . When another brick is added , average becomes 355 cm , find length of 21st brick.

Answer 1

the block is added but in the question after the block is added the length decreases so therefore it Is not possible

Answer 2

The average of the lengths of 20 bricks is 360 cm

Let the length of 21st brick = x cm

When 21st brick is added , the average reduces to 355 cm

Average = $\frac{Sum-of-lengths-of-bricks}{Total-number-of-bricks}$

hence sum of lengths of first 20 bricks = 360×20

                                                                 = 7200 cm

When the 21st brick is added, the total sum of lengths of bricks=(7200+x)cm

But their average is 355 cm

Average = $\frac{Sum-of-lengths-of-bricks}{Total-number-of-bricks}$

here total no. of bricks = 21

Hence,

355 = $\frac{7200+x}{21}$

7200+x = 7455

x = 255 cm

Hence length of 21st brick is 255 cm

HOPE IT HELPS !!