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 ?

A. D D D D D D R R R R R R

B. R D R D R D R D R D R D

C. A D D D D D

D. D D D D D A

E. D D D D D D D A

D doubles the number. A and R add or substract 1.

So in:

D R D R D R D R D R D R

Deduce that this has to be an odd number, The second to last D will make whatever number before it even. R will make it odd.

Take a look at the answer choices. Option B and C end with doubling so all those numbers will be even. Cross off.

A. After last doubling operation, there are 6 substractions. Therefore reduces even (6) from an even number (formed due to operation D) which comes out even. Cross off.

Choices D and E are odd number endings. Keep these.

There are 6 doubling operations and some subtraction but answer choice E has 7 doubling operation and no substraction. Cross off E.

D.