Use the elimination to solve this equation. 2x+3y=55 9x+4y=105

X=5 y=15 (5,15). 
This is how you do it:
-4(2x+3y=55): -8x-12y=-220
3(9x+4y=105):27x+12y=315

You add the two new equations together, canceling out the y values, and get the x value. Then, you go back and plug in the x value in the original equation to get the y value.

Let represent 1/6 of the course. Then the obstacles will appear as

{T} {T,C} {T,H} {T,C} {T} {T,C,H} [end of course]

A cone will appear with every other tire.
A hurdle will appear with every third tire.