You test the numbers one by one. First you check if it's divisible by 2, and then try 3, 5, and 7, and try every prime number until you reach the square root of that number.
你逐個數拎出嚟試，睇下俾二除唔除得開，跟住試三、五、七，逐個質數試，試到嗰個數嘅開方為止。