Answer : 33
Because....
3 pillow cases needs 1.5 yards
Then to find 1 pillow case you divide 1.5 by 3
1.5/3= 0.5
One pillow case needs 0.5 yards.
If you have 16.5 yards of fabric, you divide 16.5 by 0.5 to see how many pillow cases can be made by it
16.5/0.5= 33
Hope that makes sense