1.why we have a “base 10” counting system
The base 10 counting system is probably appeared because the ten fingers of human hands.
2.how to convert 11011011 into a decimal number
From right to left, the 1 can convert into '1','2','4','8' and so on. So 11011011=1+2+8+16+64+128=219.
3.binary numbers
They consist of only '0' and '1' and they stand differently when they are in decimal numbers.
about decimalhttps://baike.baidu.com/view/359301.htm
about binaryhttps://baike.baidu.com/view/18536.htm