In this question, we have a situation where a gas is in constant pressure but changing its volume and temperature, and the best way to solve a situation like this, is through the Charles's gas law formula, which shows the relationship between volume and temperature when the pressure is constant. The formula is:
V1/T1 = V2/T2
We have:
V1 = 10.0 L
T1 = 25°C, or 298 K
V2 = 15.0 L
T2 = ?
Now we add these values into the formula:
10/298 = 15/T2
0.03356 = 15/T2
T2 = 447 K or 174°C