Pillow case: 6, towel: 0;

Pillow case: 5, towel: 4;

Pillow case: 4, towel: 8;

Pillow case: 3, towel:12;

Pillow case: 2, towel:16;

Pillow case: 1, towel: 20;

Pillow case: 0, towel: 24;

144/24=6,24/6=4. For every pillowcase you buy less, buy four more towels.