From then on, rather than being solely a function of the money supply it was held that the general price level was determined by the money supply multiplied by the velocity of money in accordance with the famous Equation of Exchange (M * V = P * Q) **. (tsi-blog.com)
In its original and most basic form it held that the general price level would change in direct proportion to the change in the supply of money, but to get around the problem that what was observed didn't match this theory it was subsequently «enhanced» by adding a fudge factor called «velocity». (tsi-blog.com)
And since price is determined by demand relative to supply, if the demand for money can't be expressed mathematically then it is pointless trying to come up with an equation that models the purchasing power of money (a.k.a. the general price level). (tsi-blog.com)