the answer is 10 days
(after 4 days, they still needed to install 360 chairs, 600-360=240 - meaning they installed 240 chairs in 4 days. to find out how many chairs they installed in 1 day, divide 240 by 4 and you find they installed 60 chairs per day. Then divide 360 by 60 to find the remaining days needed to install the rest of the chairs, which is 6 days then add it onto the 4 days already completed)
