The cow brings to her Math class a cow-culator!
Other variations of this joke, which also has “cow-culator” as the answer, are:
What does a cow use to compute?
How does a cow do math?
How does a farmer count his cows?
If you don’t want to use “cow” as part of the question, you can also ask: What does a bull add with?
Here's another good one:
According to sources though, the cow uses cow-culators in their favourite subject: Cow-culus.
Get more cow jokes at www.simplybovine.com and find out the 10 Best Math Jokes (as nominated by Teachers and Twitter) from www.tes.com.