Explain why this is not a function

because that is equivalent to the not operator

(40+F) * time = 210 miles

( 40 - F) * 2 * time = 380, takes it twice as long to go 380 as 210 down stream

F is the flow and time it time.

solve the first for time = 210/(40+F)  sub into the second

use the quadratic on line solver

flow = 39.9

my equations turned out to be -21F^2 -19F + 32840 = 0

one real root and one negative root, discard the negative root.