there is 75% of shirts left to do, split over 5 days, you can divide 75 by 5 to get 15% each day.
As a fraction you need 3/4 to be done over 5/1 days
divide 3/4 divided by 5/1
copy, change, flip
3/4 x 1/5
3x1=3
4x5=20 that's 3/20 if you divided that into a percentage its also 15% so you know you're right.
For part 2, that's simple because you can decide if 40 shirts includes the 1/4 you already did. In that case find out what 40/4 is. That's 10. If you did 10, you need to do 30 more. That would mean that 30/5 because there is 5 days. 30/5 is 6. 6 would be for 1 day, you need to know 2. That's 12.
If 40 does not include the 1/4 you already did, and it's 40 remaining. You'd have to do 40/5 which is 8. Then 8x2 is 16.
But I think the other person is correct, it would be 40 total so it should be 12 shirts.