My "guess":

[C(4,1)*C(4,1)*C(40,1)*4+C(4,1)*C(4,1)*C(36,1)*2]/C(50,3)

(16*40*4+16*36*2)/19600=18.94%

This comes from the following flops:

1) J9x -- no T, no 6
2) JTx -- no 9, no 6
3) 6Tx -- no 9, no 5, no 4
4) 59x -- no 6, no T, no J
5) 45x -- no 6, no 9
6) 46x -- no 5, no 9