75:15

If you mean the ratio 75:15, its simplest form is 5:1 (divide both terms by gcd 15).

Quick check: 75 ÷ 15 = 5 and 15 ÷ 15 = 1.

If you want it as fractions of the total: 75/(75+15) = 75/90 = 5/6, and 15/90 = 1/6.

Need it for a specific context? Let me know.