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if were only allowed to use >>, -, + to make a number X times the constant K lets assume K is 17 and K is 20 how to make an expression for each

I was trying to think of ways to do this but I cant.

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    I got the answer look at it below that looks right2012-12-18

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If $a$ is expressed in binary $2^n\cdot a=a< where $<<$ is the bitwise left shift operator.

$17\cdot a=(2^4+1)\cdot a=(a<<4)+a$ and $20\cdot a=(2^4+2^2)\cdot a=(a<<4)+(a<<2)$