SOLUTION: A 4-square train is made with 13 toothpicks. A train is made with 400 toothpicks. How many squares are in the train? a. 400 squares b. 300 squares c. 133 squares d. 130 squar

Here is the 4-square train made with 13 toothpicks.



Notice that after the first toothpick, the green one below,



You place 3 more (red) toothpicks to form the first square.



Then you place 3 more red toothpicks to form the second square,



So after the first (green) toothpick, no matter how many squares
you have made, it always takes 3 more (red) toothpicks to make the 
next square

So the sum is 

toothpicks

If we have made n squares, we have added 3 a total of n times, so




Subtract 1 from both sides:



Divide by 3:



So we have mnade 133 squares, choice c. 

Edwin