Days are business days and a month has 21 such days. All stocks pay no dividends ?

Estimate a non-dividend-paying stocks annualized volatility (use 252 trading days) using its prices in the past nine months.

Month Stock Price per share

1 80

2 64

3 80

4 64

5 80

6 100

7 80

8 64

9 80

Then, predict the value of the stock on the 10th business day of the 10th month with approximate probability 90%.