The LCD is 30.
Think of it this way. You and I are on an assembly line checking i-pads. Your job is to quality check every 6th one and my job is to check every 10th one.
Here are the ones you will check:
6, 12, 18, 24, 30 and so on
Here are the ones I will check
10, 20 30 and so on.
Notice the first one we both check? #30 - that is the LCD of 6 and 10