Question

While only 5% of babies have learned to walk but the adage of 10 months, 75% have learned to walk by 13 months of age. If the age at which Bavaria develop the ability to walk can be described by normal distribution model, find the mean and standard deviation of the normal model

Answer 1

Mean is 12.13 months

Standard deviation is 1.29

Explanation:

We need to use z-score here

Let the mean of the distribution be a while the standard deviation be b

Mathematically;

z-score = (x-mean)/SD

We can obtain the probability from the z-score

Now, for the 10th month

The z-score for a probability of P = 0.05(5%)

Can be obtained from the standard normal distribution table and that is;

-1.645

Hence;

-1.645 = (10 - a)/b

-1.645 b = 10 - a

a = 10 + 1.645b ••••••(i)

For the 13 month, we have a proportion of 75%

The z-score corresponding to P(0.75) is = 0.674 from standard normal distribution table

Hence;

0.674 = (13-a)/b

0.674b = 13 - a

a = 13 - 0.674b. •••••(ii)

Now equate both a;

10 + 1.645b = 13 - 0.674b

13-10 = 1.645 b+ 0.674b

3 = 2.319b

b = 3/2.319

b = 1.294

So the mean a will be

10 + 1.645b = 10 + 1.645(1.294) = 10 + 2.13

So mean is 12.13