You'd need to add 10 mL of hydrogen to that volume of air to reach a concentration of 300 ppm.
A hydrogen concentration of 1 ppm can be expressed as the ratio between
This means that you can get the concentration in ppm by multiplying the ration between the volume of hydrogen and the volume of air by
In your case, a concentration of 300 ppm will require a volume of hydrogen equal to
Rounded to one sig fig, the number of sig figs given for 300 ppm, and expressed in mL, the answer will be