How do you dilute 100mL of 20ppm Fe3+ to 1,2,4,6,8 ppm, 50mL each?

mL x ppm = mL x ppm

50 x 1 = mL x 20
solve for mL. That's 2.5 mL. try it.
20 ppm x (2.5 mL/50 mL) = 1 ppm.