Problem 2 : Basic Operations

Algebra Level 4

A positive real number is given. In each move, we can either add 3 3 to it, subtract 3 3 to it, multiply it by 3 3 and divide it by 3 3 . The sum of all numbers such that after exactly 3 3 moves, the original number comes back is a b \frac{a}{b} , where a a and b b are relatively prime positive integers. Find a + b a + b .


The answer is 191.

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2 solutions

Aditya Raut
Jul 28, 2014

\bullet The operation is for 3 steps, each step has 4 choices. Thus you will seemingly have 64 64 different sets of operations.

\bullet But if your consecutive 2 operations were reverses of each other, then in their case only 1 step should give the same number and this thing is there only for number 0 0 and the 1 step to be × \times or ÷ \div

\bullet This will happen for following pairs, of consecutive functions " + + and - " , " × \times and ÷ \div ". This gives 16 16 triples of functions in which only 1 step answer arrives, the answer will only be 0.

\bullet In the above cases, if the other (1st or 3rd) operation is + + or - , along with the opposites, then there are no solutions, reduces 16 cases, leaving 32 cases to look at.

If there is only + + and × \times , then that will give non-positive solutions , thus remove the 8 additional cases.

When you have a set of the type × \times and ÷ \div at the ends, due to the middle function, either answer is 0 or you get inconsistent equation, removes 4 4 cases.

\bullet If the three operations are containing only × \times and ÷ \div then answer is 0, there are 8 cases, removing few more !

\bullet Computing the other things (i listed most of them, but did that somewhat smartly), just ignore cases that give negatives and you are left with a set of values

3 , 6 , 9 , 3 8 , 9 8 , 27 8 3,6,9,\dfrac{3}{8} , \dfrac{9}{8} , \dfrac{27}{8} .

Their sum is 18 + 39 8 = 183 8 18+\dfrac{39}{8} = \dfrac{183}{8}

Hence answer 191 \boxed{191}

I missed 9 8 \dfrac{9}{8} ... Darn.

Finn Hulse - 6 years, 10 months ago

N o t e t h a t u s i n g × o r ÷ a n d o r + g i v e s s a m e r e s u l t . J u s t o n e i l l u s t r a t i o n b e l o w . 3 x 3 3 = x i s t h e s a m e a s x + 3 + 3 3 = x . I n b o t h c a s e x = 3. S o a n o p e r a t i o n t y p e A w i l l u s e × 3 o p e r a t o r . a n d a n o p e r a t i o n t y p e B w i l l u s e 3 o p e r a t o r . A l s o n o t e t h a t a l l t h r e e o p t i o n s o f t h e s a m e t y p e w i l l n o t g i v e v a l i d s o l u t i o n . S o t h e t h r e e o p e r a t i o n s c a n b e i n a n y o f t h e t h r e e o r d e r g i v e n b e l o w . F i r s t t w o o p e r a t i o n s s a m e t y p e . L a s t t w o s a m e t y p e . F i r s t a n d l a s t s a m e t y p e . S i n c e t h e r e a r e t w o t y p e o f o p e r a t i o n s , A a n d B , w e s h a l l h a v e S I X s o l u t i o n s . A A B : 3 x 3 3 = x , 9 x 3 = x . x = 3 8 . B B A : ( x 3 3 ) 3 = x , 3 x 18 = x . x = 9 A B B : 3 x 3 3 = x , 3 x 6 = x . x = 3 B A A : ( x 3 ) 3 3 = x , 9 x 27 = x . x = 27 8 . A B A : ( 3 x 3 ) 3 = x , 8 x 9 = x . x = 9 8 . B A B : ( x 3 ) 3 3 = x , 3 x 12 = x . x = 6 S u m o f x v a l u e s ( i n b l u e ) , = 183 8 = a b . a + b = 191. Note~ that~ using ~\times~or \div~~~and~~~-~or~+~~gives~same~ result.~Just ~one~illustration ~below.\\ 3x-3-3=x~is~ the~ same~ as~\dfrac{x+3+3} 3=x.~~In ~both~ case~ x=3.\\ ~~~\\ So~an~operation~type~A~will~use~\times 3~operator.\\ and~an~operation~type~B~will~use~- 3~operator.\\ ~~~\\ Also~ note~~that~all~three~options~~of~the~same~type~will~not~give~valid~solution.\\ ~~~\\ So~the~ three ~operations~can ~ be~in~any~of~the~three~order~given~below.\\ First~two~operations~same~type.~~Last~two~same~type.~~First~and~last~same~type.\\ ~~~\\ Since~there~are~two~ type~of~operations,~~A~~and~~B,~~we~shall~have~SIX~~solutions.\\ ~~~\\ A-A-B:-~~~3x*3 -3=x,~\implies~9x-3=x.~~~\therefore~~\color{#3D99F6}{x = \dfrac 3 8.}\\ B-B-A:-~~~(x-3-3)*3=x,~\implies~3x-18=x.~~~\therefore~~\color{#3D99F6}{x = 9 }\\ A-B-B:-~~~3x-3-3=x,~\implies~3x-6=x.~~~\therefore~~\color{#3D99F6}{x = 3 }\\ B-A-A:-~~~(x-3)*3*3=x,~\implies~9x-27=x.~~~\therefore~~\color{#3D99F6}{x = \dfrac {27} 8.}\\ A-B-A:-~~~(3x-3)*3=x,~\implies~8x-9=x.~~~\therefore~~\color{#3D99F6}{x = \dfrac 9 8.} \\ B-A-B:-~~~(x-3)*3-3=x,~\implies~3x-12=x.~~~\therefore~~\color{#3D99F6}{x = 6 }\\ ~~~\\ Sum~of~x~values(in~blue),~=\dfrac{183} 8=\dfrac a b. \\ a+b=\Large~~\color{#D61F06}{191}.

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