Concept: The question tests on Alligation

Solution:
Let the volumes be V1 and V2.

Concentration of Milk in V1 = 4/7100=400/7 %

Concentration of Milk in V2 = 2/5100=40%

Concentration of Milk in the final mixture = 1/2100=50%

Refer to the diagram below

We have V1/10=V2/(50/7)

 Therefore V1/V2

=10/(50/7)

=70/50

=7/5  (option b)