Hi. This is a bit lengthy so I am attaching an image that shows it worked out step by step. I hope it helps.
Take care,
Diana
3x2.7=8.1,,, im not sure this is what you were asking for but i hope it helps
XoXo
As each chair needs 12.6667 feet lumber.So 15 chairs will need 15*12.6667=190.0005 lumber