The answer would be 16 sqrt of (2/5)
use the geometric mean formula to solve for the common ratio. disregard first the first 3 terms then make 40 to be your first term (a1) and 100 as the last term (an)
the common ratio is sqrt of (5/2)
just divide 40 by sqrt of (5/2) then
divide again the quotient of the previous term.
divide and divide until you arrive at the first term