to find the mean, you add up all given numbers and divide by how many numbers is given. so if there are five numbers, you divide the sum of all of the numbers by five.
( 1 / 4 ) * 30000 =7500 words in 1 / 6 of an hour ; 2 * 7500 words in 2 * ( 1 / 6 ) of an hour => 15000 in 1 / 3 of an hour ; 2 * 15000 words in 2 / 3 of an hour => 30000 words in 40 minutes ;