1) If a and b are positive integers such that a×a = b×b+ 15, what is the greatest possible value of a?
2) Alex expresses a number in the form 4x + 5. Brian expresses the same number as 6x − 7. If Jack wants to express this number in the form 5x - d, what would be the value of d ?
3) If all the diagonals of a decagon are drawn at how many points they will intersect ?