"subtract them" he means subtract 100 by 0.151515 which will equal 99. so thats where he got 99 from. then 99 x 0.151515 will equal 15. Try dividing 15 by 99... did you get 0.151515...? so 15/99 in fraction form is also 5/33. to check if 0.151515 = 5/33 divide 5 by 33. anyway your final answer is 5/33
hope this cleared it up a bit, if not read it again carefully until you get it. :)