4:5-0:3/8

If ":" means division, then:

• 4:5 = 4/5
• 0:3/8 = 0 ÷ (3/8) = 0
• 4/5 − 0 = 4/5 = 0.8

Answer: 4/5 (0.8)

If you meant something else (e.g., a ratio subtraction), please clarify.