So,
1. Type I profits $20
2. Type II profits $30
3. Type III profits $40
4. I/day < 100
5. Type I needs 5 hrs.
6. Type II needs 10 hrs.
7. Type III needs 15 hrs.
8. Total hrs. available: 2000 hrs.
Every +5 hrs. spent yields an extra $10.
If we use 500 hrs. to make 100 Type I stereos, we will profit $2000.
If we use 500 hrs. to make 50 Type II stereos, we will profit $1500.
If we use 495 hrs. to make 33 Type III stereos, we will profit $1320.
We should use the first 500 hrs. to make Type I stereos.
We should use the last 1500 hrs. to make Type II stereos.
$2000 + $4500 = x
$6500 = x
There must be 100 Type I stereos made along with 150 Type II stereos made.