A certain calculating machine has only three operation buttons, D, R and A. The D operation doubles the number the machine displays, the R operation subtracts 1 from the number the machine displays, and the A operation adds 1 to the number the machine displays. If the machine initially displays the number 2, which of the following operation sequences (read from left to right) would lead to the same final displayed result as the sequence -
D R D R D R D R D R D R ?
D D D D D D R R R R R R
R D R D R D R D R D R D
A D D D D D
D D D D D A
D D D D D D D A
I have the solution and I will post if there are any inquiries, however I want to see how you guys are able to do this under 2 minutes, I could not figure it out. and I don't think the solution is that great at doing it under 2 either.
While solving this under 1 miunte is possible.. typing surely isnt... I better get kudos for this
Reading question carefully is very important!
We have D, which doubles the number. A and R which add or substract 1.
Now given sequence:
D R D R D R D R D R D R
We can take a look and know that this has to be an odd number, (Note second last D will make whatever number before that even and R will make it odd).
Now quickly go through answer choices, option B and C end with doubling operation, so these numbers will be even. Eliminate.
Remaining A, D and E
Look at A. After last doubling operation, there are 6 substractions. Therefore reduces even (6) from an even number (formed due to operation D)... we get even. Eliminate A.
Choice D and E both end with odd numbers. so cant be eliminated in this way.
Again lets take a look at given sequence in question. There are 6 doubling operation and some substractions. Ans choice has 7 doubling operation and no substraction, therefore E would be much larger than original sequence.
We are left with D. Lets party with D because.......
..Ans D it is!!
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