Why Aeroplane windows are round?

Two girls were born to the same mother, on the same day, at the same time, in the same month and year and yet they're not twins. How can this be ?

Which Number is the odd one out?

A) 7145
B) 2716
C) 5321
D) 4135

If "P" means "-", "Q" means "/", "R" means "+", and "S" means "*" then find the value of the following:

21 Q 7 R 9 S 10 P 13

You can see three almost identical images below. Can you spot the odd one out from them?

What does this rebus puzzle means ?

Divide 110 into two parts so that one will be 150 per cent of the other. What are the 2 numbers?

Two friends decide to get together; so they start riding bikes towards each other. They plan to meet halfway. Each is riding at 6 MPH. They live 36 miles apart. One of them has a pet carrier pigeon and it starts flying the instant the friends start traveling. The pigeon flies back and forth at 18 MPH between the 2 friends until the friends meet.

How many miles does the pigeon travel?

Assume there are approximately 5,000,000,000 (5 billion) people on Earth. What would you estimate to be the result, if you multiply together the number of fingers on every person's left hand? (For the purposes of this exercise, thumbs count as fingers, for five fingers per hand.) If you cannot estimate the number then try to guess how long the number would be.

What mathematical symbol can be put between 5 and 9, to get a number bigger than 5 and smaller than 9?