Please help me! The reactivity of Radon-220 decreases with time.
After 224 seconds, the reactivity is 6.25% of its original value. Calculate the half life of Radon-220.

The boy was above average  by 3 SDs.

i.e. 3*SD=3*8=24 cm above the average value.

Step-by-step explanation:

Given:

3015 boys with age of 11 ,Avg height of 146 cm and Standard deviation of 8 cm.

To find:

How above Avg 170 cm tall boy is?

Solution:

The mean of 3015 boys  is of 146 cm in height,

And standard deviation is 8cm.

the height of one boy is 170 cm ,

This boy's height is 24 cm above the  average height.

i.e. 170 cm -146cm=24 cm.

Hence , standard deviation is of 8cm .

1 SD=8cm

2SD=8+8=16 cm

3SD= 8+8+8=24 cm.

The height of boy  in compare of standard deviation=24/8 .

=3

Hence  the height of the boy 3 times the standard deviation ,170cm.