Let us denote the **number of cards that Bob has** with a symbol** X. **

Similarly, let us call the **number of cards that Bill has** with
*Y*.

The problem says: "Bob has three times as many baseball cards as Bill." Mathematically, this means: **X = 3*Y. **

The problem also says: "Together they have a total of 68 cards". Again, mathematically, this means:
**X + Y = 68**.

We now have a system of equations that can be solved by the substitution method. Let us replace "x" with "3y" from the first equation into the second equation.

That is,

3*Y + Y = 68

4*Y = 68

Y = 68/4 = 17.

One can now find X by either using the first or the second equations. The result will be the same. For example, the first one gives: X = 3*Y = 3*17 = 51.

Thus Bob has 51 cards, whereas Bill has 17 cards.