rowan raised 640 last year
this year he raised 15% more
percent means parts out of 100
15%=15/100=3/20
15% of 640
'of' can be translated as multiply
15%=3/20
3/20 times 640=1920/20=192/2=96
this is 15%
he raised 15% MORE SO
640+96=this year=736
he raised 736 dollars this year