mnieman

0.25 * 0.08 = 0.02
0.5 * 0.12 - 0.06
0.25 * 0.16 = 0.04

Now add all those three to get the Expected return of the portfolio.

0.02 + 0.06 + 0.04 = 0.12

The variance formula is as follows:

Variance = 0.25 * (0.08-0.12)^2 + 0.5 * (0.12-0.12)^2 + 0.25 * (0.16-0.12)^2
Variance = 0.0004 + 0 + 0.0004
Variance = 0.0008

Standard Deviation = Variance^0.5

Therefore:
Standard Deviation = Variance^0.5 = 0.0008^0.5 = 0.0283

Hope it helps.

Good luck