Step 1: Draw a box and write the number found after the word "than" (i.e. "?") in this box.

Step 2: Draw a dot at the top-right corner of the first box. This marks the starting point of the arrow we are about to draw next.

Step 3: Draw an arrow from the dot pointing to the left. (The arrow points to the left because it is a "less than" question.

Step 4: Since it is "less than", we are actually making the unknown value "?" (starting number) smaller by 4. Hence, draw a second box directly below the arrow and the first box to show that the unknown value "?" has been shortened by 4.

Step 5: Draw a last box next to the second box to make the model form a rectangle and write the number 13 in this box.

From the model,

13 + 4 = ?

13 + 4 = 17

Thus, the answer is 17.