We know that mean = sum/ number
Given that, Mean of 4 numbers = 15
⇒ Sum of the 4 numbers = 4 x 15 = 60
Mean of the first 3 numbers = 9
⇒ Sum of the first 3 numbers = 3 x 9 = 27
Mean of the first 2 numbers = 4
⇒ Sum of the first 2 numbers = 2 x 4 = 8
Fourth number = sum of 4 numbers – sum of 3 numbers = 60 – 27 = 33
Third number = sum of 3 numbers – sum of 2 numbers = 27 – 8 = 19
Second number = Sum of 2 numbers – given number = 8-2 = 6
∴ The other three numbers are 6, 19, 33.