Agas occupies a volume of 40.0 milliliters at 20 c if the volume is increased too 80.0 milliliters at constant pressure, the resulting temperature will be equal to

Charles law gives the relationship between temperature and volume of gases. It  states that the volume of gas is directly proportional to temperature at constant pressure. 
V / T = k 
where V - volume and T - temperature in Kelvin and k - constant 

where parameters for the first instance are on the left side and parameters for the second instance are on the right side of the equation 
T1 - 20 °C + 273 = 293 K
substituting these values in the equation 

T = 586 K
temperature in celsius = 586 K - 273 = 313 °C
new temperature is 313 °C

Hope the image of the frequency formula