You can remember the meaning of 大多数 with this easy mnemonic:
The majority of teenagers have cell phones. In fact, technology is so big (大) among teenagers nowadays that many (多) teachers have incorporated it into their lessons. I've lost count (数) of how many classes I've seen that use iPads as tools.
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.