Next year's holiday entitlement is this right?

[MYA2016]

Working full time I was given 25 days holiday plus bank holidays on top.
I return in the new year from maternity leave and will be working Tuesday, weds, Thursday and Friday full days.
My colleague who returns at the same time also working 4 full days will be doing Monday - Thursday .

We have both been told we will get 20 days holiday.

I'm confused as to what this means, am I not entitled to a proportion of bank holidays on top because I don't work Mondays and most of them are on a Monday?

very relieved it's not just me!

To update ... 24.5 was correct. Seems I wasn't the biggest thicko - the girl in administration who works it out hadn't updated the formula on her spreadsheet for next year and she was very grateful i had queried it as she was in the process of sending letters out to all part time staff outlining their entitlement for 2017!
Thanks for the advice again