wahedhossain wrote:

What is the next number in the series : 5-3-1-7-3-2-9-5-.........

Want proper explanation .........................

Dear

Wahedhossain:

I send you a pm earlier in the day. First of all, this question, while intriguing, is not in standard GMAT format (PS or DS). Second, it's so open-ended --- the GMAT would never ask anything like this.

It's so open-ended --- the trouble is, the longer I stare at it, the more I can imagine various strange bizarre patterns.

Here is one possible pattern:

a) start with an odd number

b) go down to the highest prime number greater than than number's sqaureroot

c) go down the largest factor of the difference a - b that is less than b

d) go up to the next odd number above (a)

That gives 5-3-1-7-3-2-9-5-2-11-5-3-13-5-4.....

Here's another possible pattern:

a) start with 5

b) descend by subtracting 2's

c) when you get as low as you can go, go up to the next highest odd number

d) descend by subtracting 4 & 1 in alternation

e) when you get as low as you can go, go up to the next highest odd number

f) descent by subtracting 4's

g) when you get as low as you can go, go up to the next highest odd number

h) descend by subtracting 6 & 1 in alternation

i) when you get as low as you can go, go up to the next highest odd number

j) descent by subtracting 6's

etc.

5-3-1-7-3-2-9-5-1-11-5-4-13-7-1-......

As you can see, both of these "patterns" are relatively contrived, and I easily could come up with a dozen more. The point is --- there's nothing about the pattern that leaps out and taps into a clear mathematical rule. Because this is given without any context, it makes the choice of isolating one correct pattern from among many just about impossible.

If you could provide more information about the source, about any context, possible answer choices, etc. etc. that would be very helpful.

Mike

_________________

Mike McGarry

Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)