The average height of 8 adults is 1.8 m. what is the sum of their heights? how do i work this out without a calculator?

Well, the average of a sum of number is found by adding each given number and dividing that number by the amount of given numbers. 
Example: 5+4+3+8=20 
                and 20/4 = 5, the average of the numbers
Multiply 1.8 by 8.0 using this format:

1.8
x8.0


Hope this helped