A stock has an expected return of 10% pa and you're 90% sure that over the next year, the return will be between -15% and 35%. The stock's returns are normally distributed. Note that the Z-statistic corresponding to a **one**-tail:

- 90% normal probability density function is 1.282.
- 95% normal probability density function is 1.645.
- 97.5% normal probability density function is 1.960.

What is the stock’s standard deviation of returns in percentage points per annum (pp pa)?