The average of 11 numbers is 30. If the average of the first six numbers is 17.5 and that of the last six is 42.5, then what is the sixth number?

Average of 11 numbers = 30

Step 1: Calculate total of 11 numbers by multiplying it by average value 30 = 11 x 30 = 330
Step 2: Calculate the total of the first six members by multiplying it by average value 17.5 = 17.5 x 6 = 105
Step 3: Calculate the total of the last six members by multiplying it by average value 42.5 = 42.5 x 6 = 255

Therefore,
we can find the sixth number by adding the value of the first six and last six numbers and subtracting it from the total value of 11 numbers.
Sixth number = (105 + 255) - 330
= 30.