multiply average by the number of items there are since to find the mean you have to add up all the numbers and divide it by how many numbers there are. this way, we can figure out the fifth book after finding the sum of the four books and subtracting it by the average multiplied by five (I hope this made sense)
1.8x5 = 9
1.4+1.9+2.1+2.4=7.8
9-7.8=1.2
