Which country gets ill every time?

If,
29 - 1 = 30
9 - 1 = 10
14 - 1 = 15

Based on similar logic, Can you prove that the below algebraic equation is true?
11 - 1 = 10 ?

A man is looking at a photograph of someone. His friend asks who it is. The man replies, “Brothers and sisters, I have none. But that man’s father is my father’s son.” Who was in the photograph?

If you toss a coin 10 times and it lands heads up every time, what are the chances it will land heads up if you toss it again?

Two Twins Jack and Jill were standing back to back and suddenly they started running in opposite direction for 4 kilometres and then turn to left and run for another 3 kilometres.

what is the distance between the Baggio twins when they stop ?

Two trains start at the same time, one from Bangalore to Mysore and the other from Mysore to Bangalore. if they arrive at their destinations one hour and four hours respectively after passing one another, how much faster is one train running than the other?

How many letters are in the alphabet?

When it's bad luck to meet a black cat?

What occurs twice in a week, once in a year but never in a day?

My cousin lives in one story house in Antarctica. His house is made the of-of sandstone. what is the color of stairs?