a) Ladder method

10 | 1080, 1350
3 | 108, 135
3 | 36,  45
3 | 12, 15
– | 4 , 5

LCM = 10 x 3 x 3 x 3 x 4 x 5
= 5,400 buttons

b) Siti will need to buy 5 yellow packets and 4 green packets