exam Maths Puzzle Numerical Ability

I dont understand this:

Kendra invested $850, part of it at 9% and part of it at 12%. If her interest for one year was $81 how much did she invest at each rate?

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exam Other Question

the formula m=0.3048 coverts length in feet f to length in meters m. a building that is 270 feet tall, is how tall in meters? I am supposed to solve for x and the base of the logarithm is 10 but I can not figure it out.

log[10+3x]=7log4