Given Sarah could rum 800m in 3 minutes and 20 seconds = 3*60+20 = 200 seconds when she was in year 8.
Where as she could run 800m in 2 minutes 44 seconds = 2*60+44=164 seconds.
Percentage improvement in her time = (difference in time/ original time) * 100
Hence there is an improvement of 18% in her time.