In a mixture of 21 liters, the ratio of milk to water is 1:2. What is the amount of milk to be added if the ratio has to be 2:1?

:

Number of litres of milk in the mixture = \$1/(1 + 2) \times 21\$ = \$1/3 \ times 21\$ = 7 litres

So, water present in the mixture = \$21 - 7 = 14 litres\$

Let the quantity of milk to be added be 'x' litres

Then, \$"milk"/"water" = 2/1\$

⇒ \$(7 + x)/14 = 2/1\$

⇒ \$7 + x = 28\$

⇒ \$x = 28 - 7\$

⇒ \$x = 21\$

Thus, 21 litres of milk to be added if the ratio has to be 2:1.