probability of a selection

Hi,

A bag has 40 tickets number from 1 to 40. 4 tickets namely t1,t2,t3,t4 have been taken and arranged as t1<t2<t3<t4.

Then what is the probability of having t3 as 20.

What i have tried is, t1 can range from 1 to 19. so the t1's probability is 19/40.

t2 can be remaining 18 numbers which are less than 20, and total tickets are 39 now. so 18/39.

And t3 is 1/38 and t4 can be any one of the remaining 37 tickets so 1/37.

The probability of the interested event is 19/40 * 18/39 * 1/38 * 20/37.

But, this is not matching with the answer.

Please correct if my approach is wrong.

thanks.