Land measuring

Question

In karnataka land is measured in Guntas. 40 Guntas=1 acres. i want to create formula when 40 guntas is enterned it should convert into 1 acres. How to customize and insert formula in excel. Cell to be formated in numbers with 2 decimals. when 40 is entered before decimal point it should automatically take as 1. for example after 60 secs 1 minutes, after 60 minutes 1 hour like that.