How to easily change ㎡ to square footage (approximation)

If it is expressed as ㎡,

Subtract the last digit and add x3 to the first digit.

For example, if it says 40㎡, 4 x 3=12 pyeong

If the last digit is greater than 5, such as ㎡, instead of subtracting, round it up and see it as 80.

x 3=24 pyeong