Mental math means calculating the given numbers without using anything that can help you solve the problem like paper and pen, calculator, etc.
Now let’s have 81-16
=> 81 -16
=> 65
Now, let’s check if our answer is correct by adding the answer or the difference by the subtrahend.
By the way in subtraction
81 => minuend
16 => subtrahend
65 => difference
=> 65 + 16
=> 81
Our answer is correct. Hope this help.