There is a practice of calculations of some stoichiometry-based calculations. It is compulsory to run it at least once and obtain more, than zero points. The empty answer is evaluated as -1 point. So try all four types of exercises. You are allowed to do more attempts.

The next practice of calculations contains chemical equations based calculations.  It is compulsory to run it at least once and obtain more, than zero points. The empty answer is evaluated as -5 point, correct one +5, wrong result is 0. So, calculate all six results. You are allowed to do more attempts. 

See the Start restrictions...