a) 15 minutes to 2 hours: To simplify this ratio, we need to convert both quantities to the same unit. Since there are 60 minutes in an hour, we can convert 2 hours to 2 * 60 = 120 minutes. So the ratio becomes 15 minutes to 120 minutes. To simplify this ratio, we can divide both quantities by their greatest common divisor, which is 15. The simplified ratio is 1 minute to 8 minutes.

b) 125 ml : 1 litre: To simplify this ratio, we need to convert both quantities to the same unit. Since there are 1000 ml in a litre, we can convert 1 litre to 1 * 1000 = 1000 ml. So the ratio becomes 125 ml to 1000 ml. To simplify this ratio, we can divide both quantities by their greatest common divisor, which is 125. The simplified ratio is 1 ml to 8 ml.

c) 9 days : 3 weeks: To simplify this ratio, we need to convert both quantities to the same unit. Since there are 7 days in a week, we can convert 3 weeks to 3 * 7 = 21 days. So the ratio becomes 9 days to 21 days. To simplify this ratio, we can divide both quantities by their greatest common divisor, which is 3. The simplified ratio is 3 days to 7 days.

d) 6 months : 4/3 years: To simplify this ratio, we need to convert both quantities to the same unit. Since there are 12 months in a year, we can convert 4/3 years to (4/3) * 12 = 16 months. So the ratio becomes 6 months to 16 months. To simplify this ratio, we can divide both quantities by their greatest common divisor, which is 2. The simplified ratio is 3 months to 8 months.