11 (9, 18, 27, 36, 45, 54, 63, 72, 81, 90 and 99) Those numbers divisible by 9 are the multiple of 9; thus need to know how many multiples of 9 there are between 1 and 100:
100 ÷ 9 = 11 r 1 ÷ last multiple of 9 is 11 × 9 (= 99) → There are 11 - 1 + 1 = 11 numbers between 1 and 100 which are divisible by 9.
Since the ratio of red drops to yellow drops is 55:88 and you have 1515 red drops, the yellow drops needed is given as If 55=1515 88=? 88/55*1515=2424 therefore 2424 drops of yellow will be needed to make a shade of orange
