Simply divide each cost by its corresponding number of units to get the unit rate.
National Mobile = 9.99/300 = 0.0333 cost per text
Airtime = 17.99/500 = 0.03598 cost per text
National Mobile is cheaper than Airtime. It is 0.00268 cheaper per text.
Compare:
0.0333 x 500 text = 16.65 of National Mobile vs 17.99 of Airtime.
0.03598 x 300 text = 10.794 of Airtime vs 9.99 of National Mobile.