Today I did my homework and found one question that is too difficult to answer to me.

The question was like this: She decided to marry Tom. ≒She decided ( ) marrying Tom.

I’ve been studying English for 6 years. But I think I’ve almost never heard the expression “decide on doing”.(At least I don’t remember that.) Is this expression relatively common, or is this so usual?

If the two expressions “decide to do/decide on doing” are used in different ways, please tell me the difference.