The numbers are in batches of 3. The series is formed by multiplying the first number by 3 then dividing the product by 2.

10 x 3 = 30 : 30 ÷ 2 = 15

16 x 3 = 48 : 48 ÷ 2 = 24

25 x 3 = 75 : (75 ÷ 2 = 37.5)

The number required is 75

10 x 3 = 30 : 30 ÷ 2 = 15

16 x 3 = 48 : 48 ÷ 2 = 24

25 x 3 = 75 : (75 ÷ 2 = 37.5)

The number required is 75