The density of a liquid is defined as the weight per unit volume of the liquid. The densities of two liquids A and B are in the ratio $2:1$. The liquid B evaporates at a rate (in kg/hr) which is twice as fast compared to that of liquid A, which evaporates at a rate of $1$ kg/hour. If $70$ kg of liquid A is mixed with $30$ kg of liquid of B to form a mixture, find the number of hours the mixture needs to be evaporated so that the density of the resultant mixture is $1.04$ times that of the original mixture. Assume that there is no chemical reaction between the liquids
- $2.5$
- $3$
- $3.5$
- $4$