Over time, those who cannot live through the temperature differences/have undesirable qualities will die off. Overtime, those with desirable qualities are left and they will continue the population.
I'm not exactly sure what specifically you are asking for in relationship to balancing equations, but I have some examples with working out so you can examine them and try to go through the steps with them.
Hope this helps! <3
1. a. egg album, which is a translucent fluid, changes to opaque white solid after boiling
2. b. 5-carbon sugar
3. fats, phospholipids and steroids
I hope this helps you out!! <3