The sixth number is 30 if the average of 11 numbers is 30,
the average of the first six numbers is 17.5,
and the average of the last six numbers is 42.5.
To find this, we can use the following steps:
Calculate the total of the 11 numbers by multiplying the average value of 30 by 11, which gives us 330.
Calculate the total of the first six numbers by multiplying the average value of 17.5 by 6, which gives us 105.
Calculate the total of the last six numbers by multiplying the average value of 42.5 by 6, which gives us 255.
Find the sixth number by adding the value of the first six and last six numbers and subtracting it from the total value of the 11 numbers,
which gives us (105 + 255) - 330 = 30.