Grade 8 Algebra 1, Course 2 - Acid Solutions

THIS IS VERY URGENT

This isn't actually an acid solution, just something like it. I would GREATLY appreciate it if you can help me understand this

A man invested $1200 for one year, part at 4% and part at 6%. He earned 58$ from his investments. How much did he invest at each rate?

The way I learned it was doing something like this

1...|2.........|New

.04 |.06......|.1

x....|1200-x.|1200

.04x|72-.06x|120

-.02x = 48

I know I'm doing something wrong there - The chart may look screwed up, but it was multiply down and add across.

EDIT:

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I'd also like some help on understanding this:

Jake can run downhill at a rate 2 times faster than he can run uphill. If the distance each way is 5km, and the total time for the trip is 30 minutes, what is his rate downhill?