Hi All,
We're told that three digits have been removed from each of the following numbers and n = 25. We're asked which of the numbers is equal to 3*2^(n-1).
To start, the fact that three digits have been removed from each answer is irrelevant. When a GMAT question presents information that appears to be a gigantic calculation, there is almost always a pattern behind the numbers. Here, the pattern is the 'units digit'; each of the 5 answers has a different units digit - and we'll use that, along with some basic Arithmetic, to answer this question.
(3)(2^24) = a big number
The GMAT won't expect us to physically calculate that number though, so let's look for a pattern in 2^24....
2^1 = 2
2^2 = 4
2^3 = 8
2^4 = 16
2^5 = 32
2^6 = 64
2^7 = 128
2^8 = 256
Notice the pattern that exists in the units digits: 2....4....8.....6.....2.....4....8....6
This means that for every 4 increasing "powers of 2", the same pattern will repeat. 2^24 would give us 6 sets of those repeating four digits, meaning that 2^24 will have a units digit of 6.
When we multiply a number that ends in 6 by the number 3, we end up with a product that ends in 8. There's only one answer that matches....
Final Answer: E
GMAT assassins aren't born, they're made,
Rich
