Final answer:
The substance will take 60 years to decompose to 10% of the original substance.
Step-by-step explanation:
The half-life of a substance is the time it takes for half of the substance to decay. In this case, the half-life is 20 years. To find out how long it will take for the substance to decompose to 10% of the original substance, we can use the concept of half-life. After one half-life (20 years), the substance will have decayed to 50%. After two half-lives (40 years), it will have decayed to 25%. After three half-lives (60 years), it will have decayed to 12.5%. And after four half-lives (80 years), it will have decayed to 6.25%. So, it will take 60 years for the substance to decompose to 10% of the original substance.