Many people are very good at day to day mathematics, especially if they are in constant touch with addition and subtraction. They are very fast in calculating in their mind. When we learn maths in our senior KG and 1st standard, we are taught to add two numbers by keeping one number in the mind and one on the finger and then just add up.
Like you add 5+6, we are taught, 5 in the mind and six on the fingers. After five, six, seven, blah blah eleven. Bang there is the answer. Sounds funny now, but that is how we learn. As days go by we master addition and subtraction. But the trouble comes when we are adding fractions. Which is kind of tricky to calculate in mind.
Something similar used to happen to the guy who sells milk early in the morning near my house. He sells lot of half a litre packets. So he has to always check the money in 3 and half litres or like 5 and half litres. So the half here and there used to create problems to his less educated brain. I have to say, these guys can suprise you with their math. They are very quick in calculating when it is whole numbers. Some guys are very good at fractions too.
Usually at my house, we take 3 half litre packets per day. Each half litre packet cost Rs. 8/- . Hence we take one and half litre per day of milk. Some day if there are guests or may be some festival we buy more packets. He writes it on his diary and my mom also marks it in her record and we pay him monthly.
Once he was home and brought his whole list which he maintains to calculate. He used to see it litre wise and calculate money according to that. It took him lot of time to finish calculation and come to an amount due that month every time. His calculation seemed very cumbersome. My mom calculates number of packets. As she says, "When there is fractions I cannot calculate fast and I get confused". So the number of packets can never be a fraction. :) :)
She told him how to calculate packet wise. For the milk guy it was a total paradigm shift, he suddenly started to calculate faster than he thought he could. Delighted he took the amount for that month and left happily. We live on the ground floor. As it is 3 storey building, he used to collect money on the same day in the start of every month after he has collected from us. Once he collected money due for that current month and went up to collect money from other flats. He came back in just 15 mins. Suprised my mom asked him, that was very fast. He gave a shy smile back which showed lot of gratitude and thankfulness. Indeed common sense and simple techniques work wonders.
Some days back I had given a booklet for xerox. The xerox guy told me some amount and each copy cost 75 paise. This is again odd for calculation. I started to count the number of pages. He stopped me and showed me the page numbers in the booklet. I was taken aback by my common-sense quotient. I have a lot to learn from these people. And I thought my math was good after I scored 100 in one of the exams in my school days. :) :)