To solve this, set up an equation for how many hours you can stay up per coffee. To do this, first figure out how many hours you can stay up per coffee by dividing your number of hours by your number of coffees, in this case 2 hours/4 coffees, giving you 1/2 hour per coffee. Now, set up your equation to be h=rc, with h being your hours, c being your number of coffees, and r being your ratio, which you determined to be r. You now have h=(1/2)c, which you can plug 36 coffees into to get h=(1/2)(36)=18 hours. This means that if you drink 36 coffees, you can work 18 hours straight.
