What's the next in the series?

H, Be, F, S, Mn, Kr, In, Gd, Tl, ?

What number comes next in this sequence:
7 8 5 5 3 4 4 _

Replace the question mark with the number below given a simple math problem.

Nine marbles are arranged in the image below. Can you slide two marbles to form a square?

In 1990, a person is 15 years old. In 1995, that same person is 10 years old. How can this be?

John, a 5-year-old boy, was really fond of the chocolates. He asked his Mother to give him some money to buy his favourite chocolates. His Mother gave him $45. He went to the shopkeeper and asked, "How much is one chocolate for?". The shopkeeper said $3 for one chocolate. Also, if you give me the wrappers of three chocolates, I will give you one for the exchange.
In total, how much chocolate could John eat?

How high do you have to count before you use the letter “a” in the English spelling of the whole number?

What always comes into a house through the keyhole?

Detective Rockford was jogging near the beach at 4:30 am.
He hears a sound near the shack "No Michael, Please Do not shoot me".
Next instance he heard the sound of gunfire. Rockford rushes to the shack where he finds women lying dead and a gun in close proximity of three "Doctore Lawyer and a Teacher".

Rockford immediately knew that the Lawyer has committed the crime. How?

In the given figure, move three matchsticks so that the resulting figure contains two rectangles.