In P.E. class, Xiaoming used to run 100 meters in 18 seconds, but now it only takes 15 seconds. What percentage has his speed increased?
(100/15-100/18)/(100/18)*100%=(1/15-1/18)/(1/18)*100%=1/5*100%=20%
A: increase by 20%
(100/15-100/18)/(100/18)*100%=(1/15-1/18)/(1/18)*100%=1/5*100%=20%
A: increase by 20%