ˌmental aˈrithmetic BrE AmE noun [uncountable]
the act of adding numbers together, multiplying them etc in your mind, without writing them down:
I did a quick bit of mental arithmetic.
ˌmental aˈrithmetic BrE AmE noun [uncountable]
the act of adding numbers together, multiplying them etc in your mind, without writing them down:
I did a quick bit of mental arithmetic.
Longman Dictionary of Contemporary English. Longman - Словарь современного английского языка. 2012