A Turing machine can compute product of any two numbers, hence decidable problem. However, if the question is asked to find out whether a given Turing machine can perform product of two numbers, then it is an undecidable problem
[ Proof: design a (halting) Turing machine which can compute product of two number (recursive & decidable). Now give the same input to the given Turing machine and compare the output with your TM's output. If you can enumerate this to all the natural number then you can say the given TM can perform the product of two number. Since the given TM is unknown to us, it could be possible that for any two number given as a input to the TM might cause it to loop infinitely and it might never halt (halting problem). If that happens, then the entire problem of whether a particular TM can perform product of two numbers will become undecidable. ]