the LCD is the LCM (least common multiple) of the two number which is 105 in this case
LCD(21, 13, 14, 15) = 2730
It is: 105
20
LCD(49, 21) = 147
LCD(9, 21) = 63
LCD(18, 21) = 126
LCD(9, 15) = 45
LCD(15, 26) = 390
9 = 3^2 15 = 3 x 5 21 = 3 x 7 lcm = 3^2 x 5 x 7 = 315
21
LCD(15, 7, 8) = 840.
15