Mixtures and combined rates word problems

Dear Khan Academy, I love your site and have used it for the past few years to get confident at math.

I cannot work out the problems in Precalculus unit 4 Modeling with rational functions. I feel like I have a pretty strong grasp of Algebra yet I feel that I do not have tool in my toolkit to know how to even approach the problem. I heard it suggested that these problems were more geared towards a physics course and hopefully I can come back in a few years with better skills. 

I wonder if your team feels the same way as I do. Any suggestions or help would be appreciated. Thankyou.