Fast math multiplication

Also uploaded my drill sheet.. If you solve 21, send me a private message.

I think I solved this correctly, if I didn't, someone please let me know. So you're given that 6 people can lay a certain amount of bricks in 10 hours. The problem doesn't tell you how many, but that's ok. Let's make it easy and say that 6 people, in 10 hours lay 60 bricks (set this as the goal amount). So that's 6 bricks per hour between 6 people, so 1 brick per person, per hour. At 5PM, 1 person is added every hour. From 11AM to 5PM is 6 hours. So 36 bricks have been laid. From 5PM to 6PM, 1 person was added, to give 7 total. We know each person lays 1 brick per hour, so by 6PM 43 bricks have been laid (36+6 original people+1 new added person). 6PM to 7PM one more person is added, so a total of 8 people. By 7PM, 51 bricks have been laid (43+6 original people+1 previously added person+1 new added person). Finally from 7PM to 8PM one more person is added, a total of 9. And the total amount of bricks laid is 60 (51+6 original people+2 previously added people+ 1 new added person). So from 11AM to 8PM is 9 hours to complete the job. Once again, I hope this is correct..