Is my solution good or a fluke?

[dux3000]

If f(x) = x^2 + 5x + k and f(2) = 6, then f(4) = 

12

26

28

36

38

The solution (here http://www.greenlighttestprep.com/module/gre-algebra-and-equation-solving/video/1013 ) finds the value of k and then figures out f(4).

I didn't do that. Here's what I did.

2^2 = 4 and 4^2 = 16. 16-4 = 12, so I know that f(4) will be 12 more than f(2)

Also, 5(2) = 10 and 5(4) = 20. 20 - 10 = 10, so I know that f(4) will be 10 more than f(2)

In both functions we're adding k both times, so there's no difference there. 

So 12 + 10 = 22, so  f(4) will be a total of 22 more than f(2)

f(2) = 6, so f(4) = 6+22 = 28 (answer c)

Is this a valid solution or did it only work  because the numbers worked out nicely? 

 

[Vince Kotchian GRE Prep]

Sure, it's valid. Nice logical thinking... although it's probably easier to do it the first way.

[dux3000]

Wonderful! Thanks!

 

[GREMasterEMPOWERRichC]

Hi dux3000,

Most GRE Quant questions can be approached in a variety of ways, so the 'measure' of any approach ultimately comes down to two things:

1) Did it help you to get the correct answer.

2) Did it help you to do so in a 'relatively fast' way (so that you can get to all of the questions in the section without having to rush through any).

If your approach does both, then there shouldn't be a concern. However, if you're getting too many questions wrong or if you have a pacing problem, then there might be some fundamental problems with 'your way' of approaching the Quant section.

GRE Masters aren't born, they're made,

Rich