Answer:
70db
Explanation:
If one typewriter 's sound recorded 60 dB, therefore 10 typewriters that clack away would register 70 dB (doesn't seem 600 dB!), but it would only sound double as loud as compared with one typewriter. You would gathers typesetters to strike a noise level of 80 dB and jointly they will only sound four times as loud as compared single typewriter.
So, it would be
= 20 + 50
= 70db