1,950 ml of water should be added to 780 ml of sodium chloride to maintain the ratio of 2:5
Given:
Ratio of sodium chloride to water 2:5
sodium chloride = 780 ml
water = ?
proportionality a:b = c:d where ad = bc
a = 2 ; b = 5 ; c = 780 ml ; d = x
2:5 = 780:x
2x = 780*5
2x = 3900
x = 3900/2
x = 1950 ml of water.