Answer :
at the start of 2nd day, he completed 4 chairs....at the end of the day, (9 hrs later) he completed 7 chairs....that means he had completed (7-4) = 3 chairs in 9 hrs. 9/3 = 3...that means it takes 3 hrs to complete 1 chair.
He has 7....so he needs 3 more chairs...and if 1 chair takes 3 hrs, then 3 chairs takes (3 * 3) = 9 hrs <===
He has 7....so he needs 3 more chairs...and if 1 chair takes 3 hrs, then 3 chairs takes (3 * 3) = 9 hrs <===