9514 1404 393
Answer:
about 100 years
Step-by-step explanation:
2020 -1920 = 100
Outdoor lights were made safe about 100 years ago. (using information given in the problem statement)
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Additional comment
2020 -1930 = 90. So, the actual length of time is between 90 and 100 years.
Using additional research, we find mercury vapor lights were made to have a practical color in about 1929, so 91 years ago. They were invented somewhat before the 1920's but had a greenish color. Fluorescent phosphor coating was added in 1929 to make them produce a whiter light.
