Hi

 

Milk gets sour at room temperature and is more faster at a bit higher temperature ie 30-40 . Till this temperature we can assume that the lifetime of milk L is inversely proportional to temperature. This can be seen from the fact that milk kept at low temperature lasts longer. ( not for many days) .

Now

             L  1/T

                           L₁*T₁ = L₂ * T₂

ie  3L * 3 = 20 * 64  ( given lifetime of milk at 3 degrees is 3 times the total)

  L =  1280/9.

 

Using in second case.        3 * 1280/9 = 40 * L₄₀

                  L₄₀ = 10.66 ~ 11 hours .

I dont know whether this approach is right. Friends pls check it.

 

Cheers!