Holiday Homework

So we did some homework again and we didn't get full marks due to my answer that includes a full stop after the abbreviated Mister (Mr). There is no full stop after "Mr" anymore. Am I so old-school, really? 

I wiki searched-- it says a full stop is not used on abbreviations like "Mr" because the first letter (M) and last letter (r) are included in the abbreviation.

Wa..... like that win liao lor. 

Yes, really, I am outdated and I have to learn new things. No biggie, just learn.

 Changes I need to learn & adapt to.

Abbreviations and personal titles of address

According to the Oxford A–Z of Grammar and Punctuation, "If the abbreviation includes both the first and last letter of the abbreviated word, as in 'Mister' ["Mr"] and 'Doctor' ["Dr"], a full stop is not used."^[13][14] This does not include, for example, the standard abbreviations for titles such as Professor ("Prof.") or Reverend ("Rev."), because they do not end with the last letter of the word they are abbreviating.
Among Americans, however, the common convention is to include the period after these abbreviations.^[14]

 Math questions