.558 is rounded to the nearest thousandth
3 CD → $29.98
29.98 - 6.44 = 23.54 → remained
If each CD costs the same amount ... ↓
23.54 ÷ 3 = 7.8466
$7.8466
From 10 to 15 -> 10 increased with half its size (5), that means increase of 50%
From 16 to 12 -> decrease of 4, which is a quarter of 16, that means a decrease of 25%