I have been doing this for quite a number of years. I normally put in RM15.90 in an ang pow to be given to my kid who is still studying. In Cantonese, 159 sounds like 'yat mm kau' ie 'don't be playful in studies'.
But this year, I gave more ie RM159.
Put the cash inside a lovely ang pow for my kid to put it under her pillow