A subordinate clause, or a dependent clause is a group of words that cannot form a sentence.
Now ask yourself, can 'I wrote the number on a notepad' stand alone ? Yes, it can.
Now an independent clause, is a group of words with a subject and a predicate, and it can stand alone as one sentence.
Since 'I wrote the number on a notepad' can stand alone, it is considered as an independent clause.
I hope that makes sense. ^_^
