The probability for an event to happen, P(x), can be computed by P(x) = number of favorable outcomes/number of possible outcomes.
That being said, in this probllem our number of favorable outcomes is 2, having 2 T's in the problem. And the number of possible outcomes is 10, having 10 letters in the word arithmetic.
Thus, P(x) = 2/10 = 1/5 = 0.2 = 20%
Hope this helps :)
p.s. If there is a part of my solution that you did not understand, please do not hesitate to contact me and I'll be more than glad to explain it to you thoroughly