I think of n number multiply it by 3 and subtract 9 my answer is 51. what number was i thinking of?

To find the answer, you must simply work backwards.

So add 9 to 51, and divide your answer by 3.

The answer is 20

As with all questions like this, you must work backwards.

If (n x 3) - 9 = 51, you must start at the end, in this case - 9.

51 - 9 = 46. Therefore n x 3 = 46.

Then you must divide 46 by 3 to find n. 46 : 3 = n.

46 : 3 = 15.3 recurring.

Therefore n = 15.3 recurring