Question 14061
Suppose:
x is the first number
y is the second number
z is the third number

We can construct mathematic model from the problem:
x + y + z = 6
z=x+y
x=z+1

Look the first equation and the second equation:
x+y+z=6
(x+y)+z=6
z+z=6
2z=6
{{{2z/2=6/2}}}
z=3

Substitute z=3 to x = z+1
x = z+1
x = 3+1
x=4

Substitute x=4 and z=3 to z=x+y
z=x+y
3=4+y
3-4=y
y=-1

Thus, the first number is 4, the second number is -1, and the third number is 3
OK