How do we add?

Give you two number, let's say 34 and 72. How do we add? We put these two numbers together, we align them by column, we then add 4 and 9, we get 6,.... Wait a minute, are we at school or a supermarket? When we are at school, we will add 4 and 9 first, but when we are at supermarket, we will do 30+70=100 first, so if this is $34+$72, we know it is over $100..
Why do we do it differently?

How do we multiply?

Give you two number, let's say 34 and 9. How do we multiply? We put these two numbers together, we then multiply 4 and 9, we get 36,.... Wait a minute, are we at school or a supermarket? When we are at a supermarket, we will do 30 x 9=270 first, so if there are 9 items of $34, we know it is over $270, because of the $4, we know it is probably around $300.
Why do we do it differently?

This is strange! Why?

When we learned addition or multiplication at school, we always start to add or multiply from the last digit. But in real life, we always start from the first digit. When we write a number, say 356, we write 3 first. When we say it, we will say: "three hundred and fifty six", we say number 3, the first digit, first!
Then why do we add or multiply from the last digit first?