X = W + Y + Z
W - 5 = Y
Z = W - 1
What is the 4-digit number XWYZ?
We can approach this by listing all the possible values starting with Y.
Y must be 0, 1, 2, 3, 4 since Y + 5 = W and the largest number that W or any of the numbers could be is 9
Therefore, we know that W has to be 5,6,7,8, or 9.
One we know that, it's easy to see that Z must be either 4,5,6,7, or 8 since it's 1 less than W.
X is the other letters added together. The only possible number X can be is 9, obtained when W = 5, Y =0 and Z = 4. Any other combination of those numbers will make X greater than 9.
The 4-digit number is 9504.