Suppose that y=r+x, where 0≤x<1 and r is an integer.
Then ⌊y⌋=r.
r(r+x)=42x=42r−r
Then x has to satisfy the inequality 0≤x<1.
We find the range of values of r such that 0≤42r−r<1.
Since y > 0, r >= 0. We multiply each term by r to get 0≤42−r2<r.
Solving 0≤42−r2 gives −√42≤r≤√42.
Solving 42−r2<r gives r>6 or r<−7.
Combining the solutions of the two inequalities gives 6<r≤√42. But this inequality has no integer solutions.
Therefore, there are no such value of y such that y * floor(y) = 42.