How do i solve it it doesn't make sense if 6+4. =10 and 8+5=13 the how do i find the sum if it's greater than 10 and less than 13

answer:  

\boxed {\frac{1}{5}}  

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solution-

60 *   \frac{1}{5}   = 12

90 * \frac{1}{5} = 18

75 * \frac{1}{5} = 15

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hence, the answer would be \boxed {\frac{1}{5}}