You might have seen so many puzzles on math which can be solved in seconds but this is the one which gives a tough competition for all such problems.
can you make 100 by using exactly four 7s'??
7,7,7,7 => 100
Think beyond everything and try to get the solution.
If you give an extra "one 1" then we can do it ( 7 + 1/7 ) * ( 7 + 7)
since you missed the one I think we can't get it.
It is possible even without "1"