What volume of 0.20 mol dm -3 NaOH solution would yield 5.0 g of NaOH...

What volume of 0.20 mol dm ^-3 NaOH solution would yield 5.0 g of NaOH on evaporation to dryness? [NaOH = 40g mol^-1]

The correct answer is: B

Conc. g/dm³ = 0.20 x 40 gmol ^-1 = 8 g/dm³
1000 cm³ of 0.20m NaOH contains 8g of NaOH :. 5g of NaOH will be contained in 5/8 x 1000 x 625cm³