Question

a certain quantity mixture of milk and water in the ratio 4:1 after adding 20 litres of water the ratio becomes 5:7

Answer 1

according to question let water be w and milk be m
$\frac{m}{w} = \frac{4}{1}$
m = 4w
also adding 20litres the ratio become 5:7
$\frac{m}{w + 20} = \frac{5}{7}$
put m =4w in above equation
$\frac{4w}{w + 20} = \frac{5}{7}$
by cross multiplication
$7 \times 4w = 5 \times (w + 20)$
$28w = 5w + 100$
$23w = 100$
$w = \frac{100}{23}$
$w = 4.34litres$
m =17.36 litres