Though both are, indeed, correct, the ratio of usage of learned vs. learnt in American published works in the last 200 years, is more than 8 to 1 (using Ngram from google books).
Though both are, indeed, correct, the ratio of usage of learned vs. learnt in American published works in the last 200 years, is more than 8 to 1 (using Ngram from google books).