$$\text{%V/V} = (\frac{\text{Volume of solute}}{\text{Volume of solution}}) \times 100$$

In this case, the volume of solute (ethanol) is 45 mL and the volume of solution (ethanol + water) is 45 mL + 125 mL = 170 mL.

So, the percent by volume of ethanol in the solution is:

$$\text{%V/V} = (\frac{45 \text{ mL}}{170 \text{ mL}}) \times 100 = 26.5\%$$

Therefore, the percent by volume of 45 mL ethanol dissolved in 125 water is 26.5%.