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MissionGMAT's solution is perfect. However, some students benefit from organizing the information in a more visual manner. This is where the Double Matrix Method comes in.
For questions involving a population of things (this time a population of clients) where each thing has two characteristics associated with it (here the 2 characteristics are: 1) having/not having credit card, and 2) having/not having cheque book), we can solve the question using the Double Matrix method.
For a free lesson on the Double Matrix Method, watch video #20 at GMAT Word Problems  GMAT Prep NowAfterwords you can tackle the practice questions #21 and #44 on the same page.
Cheers,
Brent