In an 8hr day mark can sell 15 flowers ,lou can sell 6 flowers and cindy can sell 8 flowers. what is the smallest number of 8hr days needed for each of them to achieve the same number of sales?

Mark = 15 flowers
Lou = 6 flowers
Cindy = 8 hours

Find the Lowest common multiples 

Method 1 (Simple but tedious):
15: 15, 30, 45, 60, 75, 90, 105, 120
  6:   6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78, 84, 90, 96, 104, 120
  8:   8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120
Therefore:
15 x 8 = 120
6 x 20 = 120
8 x 15 = 120

Mark needs 8 8hrs
Lou needs 20 8hrs
Cindy needs 15 8hours

Method 2:
15 = 5 x 3
6 = 2 x 3
8 = 2 x 2 x 2
LCM = 2 x 2 x 2 x 3 x 5 = 120

120 ÷ 15 = 8 
120 ÷ 6 = 20
120 ÷ 8 = 15

Mark needs 8 8hrs
Lou needs 20 8hrs
Cindy needs 15 8hours