MATH
posted by KATE .
HOW MANY DIFFERENT ARRANGEMENTS ARE THERE OF THE LETTERS IN TESTERS?

This is a permutation.
n! (n factorial) = n(n1)(n2)...1
What is your n?
http://en.wikipedia.org/wiki/Permutation
I hope this helps. Thanks for asking. 
My solution assumed that each letter — even if repeated — was considered different. Thus T1 is considered different from T2 and so on.
I hope this helps a little more. Thanks for asking.