When did Ray Kroc invent the multimixer?

Answer 1

Ray Kroc, at 52 years old, invested his entire life savings to become the exclusive distributor of a milk shake maker called the Multimixer.

(see the McDonalds link below)

So, Ray Kroc did not "invent" the multimixer. Had he done so, he would not have to become the "exclusive distributor".

I believe the actual inventor (s) were Earl Prince and Ted Fredenhagen. Here is a nice history of Prince Castle/Cock Robin...

(see the rblandmark link below)