The neutron will turn into a proton if four coins land heads up. Take the neutrons present at 1 second and for version 2 of the Big Bang determine if each neutron makes it to 50 seconds and mark it in if it does. Evolve your set of neutrons to 200 seconds this way. You should have a reasonable number of neutrons left at 200 seconds. At 200 seconds the Universe is cool enough that deuterium can form, a necessary step in forming helium.
All deuterium is converted quickly into helium.
If you have any left over neutrons draw a circle around it and one proton. This is a deuterium nucleus. Once neutrons are inside a nucleus they stop decaying. Repeats step 2-3 for version 3. This time assume deuterium is able to form at 150 seconds so circle the helium then.