If you add 15 to 3 times the mystery number you get 78 was the mystery number

So the equation looks like this : 15*3x=78
subtract 15 from both sides:        -15      -15
then divide by 3 from both sides
to isolate the variable:                  3x=63
                                                      3/3x=63/3
                                                        x=21 (your mystery number is 21)