To start your wash, insert three tokens.  Because the timer reads tokens differently than quarters, it will read 15 on the readout.  After a short hesitation, this will turn into your actual time, 5:15.  

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Wash as you normally would.  If you need more time, you can add another token which gives you another 1 minute 45 seconds.  Or, if you don't need that much more time and don't want to spend another token because you just need a few more seconds to finish up, you can insert quarters to gain another 21 seconds each.