Let's be smart about these problem types. In fact, once I share this, you will pray you get a problem like this on test day.
The question asks us for the LEAST number of sales Ken must make to break even. Knowing that the makers of the GMAT will try to trap us with the wrong answer, we can quickly narrow the correct answer to one of two choices: C or D. We can assume without even doing any math that the answer will be some fractional answer. The trap here is that we round down instead of rounding up like the question asks us to do by asking for the LEAST number of sales he must make.
Without even putting pen to paper, I can be 99% confident that the answer is D, which is the larger of the two answers since the question asks for the LEAST number of sales he must make.
Now -- if I am short for time on test day, then this can be a fabulous way of making up time. If I have time to work out the answer that I can have 100% confidence in, then I will do the algebra, as others have stated.
Happy GMAT Hunting...