#Mathematica <s>82</s> 72
n_~u~k_:=(c=1;m=1;While[m <= n,If[FactorInteger[c][[-1,1]]<=k,m++];c++];c-1)
**Usage**
u[1, 5]
u[5, 2]
u[20, 3]
u[57, 5]
u[66, 13]
u[100, 5]
u[1000, 25]
>1
16
96
324
110
1536
5474
----------
Prime factors and their powers are in the center column.
Table[{t, FactorInteger[t], u[t, 5]}, {t, 15}] // Grid
![data][1]
[1]: https://i.sstatic.net/Va4EP.png