I purchased 7 mangoes from a nearby fruit seller yesterday as there were that many persons in my home. There are plenty of fruit sellers around, so there is no reason of purchasing more and storing. The fruit seller was a lady in her seventies and probably had no formal education. She sold those mangoes at ₹140/kg. She weighed the mangoes selected by me. It was 1 kg 750 gm. She asked me to pay ₹245 while transferring the mangoes into my bag. I was surprised as the time taken by her to do this calculation was practically nothing and she didn’t use calculator or pen-paper. I took more time than her to do this calculation manually and found the amount to be right. I thought, she might have got another customer earlier purchasing same quantity of mangoes, so she might just remember the amount! But, it was not so. I was curious to know her process of doing the calculation. She said; 14*7 = 98 for 700 gm, +₹7 for 50 gm = ₹105 for 750 gm. Add 140 for 1 kg. So, 140+105=₹245. W