$*3\mathrm{.}19$ A manufacturer blends lubricating oil by mixing $300k\frac{g}{m}in$ of No$.10$ oil with

$100k\frac{g}{m}in$ of No$.40$ oil in a tank. The oil is well mixed and is withdrawn at

the rate of $380k\frac{g}{m}in.$ Assume the tank contains no oil at the start of the

blending process. How much oil remains in the tank after $1hr?$