Ares to Mẫu (Northern Vietnam): the quick answer
1 a = 0.0277778 mẫu-b
1 are equals 0.0277778 mẫu (northern Vietnam)
To convert ares to mẫu (northern Vietnam), multiply the value by 0.0277778. To go the other way, multiply mẫu (northern Vietnam) by 36 to get ares.