If the first number is x, then the second number is x+1 and the third number is x+2. Since they add up to 78, then x+x+1+x+2=78 and 3x+3=78. If we subtract 3 from both sides, we get 3x=75 and if we divide both sides by 3 there we get x=25. Since the second number is x+1, we add 1 to 25 to get 26