Because it takes 4 days to make 20 small frames, it takes 8 days to make 40. And because it takes 3 days to make 21 frames, and 21 multiplied by 3 is 63 (one less than 64), it would take 9 days (3 times the amount for 21) to make 64 large frames. In all, it would take APPROXIMATELY 17 days (9 +8 is 17)
