A mistake made by many of them

A SMALL PUZZLE

A group of four people has to cross a bridge. It is dark, and they have to light the path with a flashlight. No more than two people can cross the bridge simultaneously, and the group has only one flashlight. It takes different time for the people in the group to cross the bridge: 
Annie crosses the bridge in 1 minute,
Bob crosses the bridge in 2 minutes,
Raju crosses the bridge in 5 minutes,
Dorothy crosses the bridge in 10 minutes.

          How can the group cross the bridge in 17 minutes?

a coins puzzle

There are 12 coins. One of them is false; it weights differently. It is not known, if the false coin is heavier or lighter than the right coins. How to find the false coin by three weighs on a simple scale?

a small puzzle

A small child, want to buy some  remote car  . he went to his mother and asked for fifty rupees ,also went to his father and asked for fifty rupees . the remote car was of cost  97 coins.   so the shop keeper in return give him 3 coins. so gave 1 coin to his mother and one to his father . the  money that was given by father and mother is 49 coins and he has one coin. the sum of all is 99 coins.where is the one coin missing?

can u guess the missing number in the series

99   ,79   ,      ,    59   ,  49?

a small question

IN OLDEN DAYS WITHOUT HAVING ANY INSTRUMENT, A MAN FIND PYRAMIDS HEIGHT. HE WAS A MAN WHO IS FEEDING SHEEP AT THAT TIME. LATER HE BECAME A SCIENTIST.CAN YOU GUESS HOW HE FINDED THE HEIGHT OF A PYRAMID?

A QUESTION ON HISTORY

ALEXANDER  THE GREAT WARRIOR HAAS OCCUPIED THE ENTIRE PERSIA IN HOW MANY YEARS FROM IS POSESSION? 

                         A) 10 YEARS                                                 C)5 YEARS

                         B)  0 YEARS                                                  D)7 YEARS