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TSO
The person buys a total of 20 frames.
 'f' of the frames are size 4 by 6, and cost 5.8
that means 20 - f frames are size 5 by 7, and cost 9.6
And the total is 80.8

So... we get:

5.8f + (20-f)9.6 = 80.8

Answer is B
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