Explanation:
A post-test or a mastery test can be unlocked if the learner who has the of it previously can complete the given tutorial with the test.
After the tutorial with the test, the learner will have to complete everything and if he passes the test with a completed answer then he will see that the Mastery Test has been unlocked. Also after the mastery test is unlocked it can be done again.
As metal + acid will form hydrogen gas and salt , the chemical equation is :
Mg + 2HCl --> H2 + MgCl2
