Tip: Don't try to manipulate inequalities yet. Leave the C (cows) and S (sheep) as they are in equality form (i.e. don't move a number over and do it to the other side). It's a hard habit to break when you're used to algebra and solving but it will save you time if you don't waste it on unnecessary note taking. Remember, data sufficiency is about seeing if you can solve, don't try to solve. Just read each option aloud to yourself and compare. Remember the goal of the question, which is a greater number, cows or sheep?
Option A: Says C < 5S. Pick a number; make the number of sheep equal to 100, S = 100. Plug in the 100 to see the numbers available to make C less than 5S or 500 sheep, 100*5. C could be any number less than 500. It could be 1 cow or 499 cows versus the 100 sheep you started with. Not enough information.
Option B: Says 1/5 * S < C. Again use 100 as your baseline for the number of sheep (S) to make it comparable to option A. 1/5 of S is 20 (1/5 * 100), therefore 20 < C. With this option, the rancher has more than 20 cows, 20 < C. Draw a number line and look to see if you can have more than one answer. If so you don't have enough information to give a definitive answer. The cow count can be 21, less than 100 sheep, or it could be 1 million cows, more than 100 sheep.
Not enough information.
Option C: Combine the options together. You know have a number line for the ranges of C. C is greater than 20 but less than 500, or 20 < C < 500. Use the 100 sheep as your baseline again. C can either be 21 cows and 499 cows and either number can satisfy the number line. Combining options makes it still uncertain as to which animal the rancher has more of. Not enough information.
Option D: Neither A nor B can be used to answer the question.
Option E: Your answer. Not enough information given in this question to answer.
With practice you'll recognize how to solve data sufficiently problems faster and faster. You have what it takes to conquer the GMAT!)