Teaser 2930: Odd socks

From The Sunday Times, 18th November 2018 [link]

I had a drawer containing some black socks and some white socks. If I drew out two socks at random the chance of getting a black pair was 1 in __.

After many washes all the socks looked grey. So I added some red socks to the drawer. Then if I drew out two at random the chance of getting a grey pair was 1 in __.

After many washes all the socks looked pink. So I added some green socks to the drawer. Then if I drew out two the chance of getting a pink pair was 1 in __.

After many washes all the socks looked brown. So I have now added some yellow socks to the drawer giving me a total of fewer than fifty socks. Now if I draw out two the chance of getting a brown pair is 1 in __.

The gaps above consist of four different prime numbers.

If I draw out two socks at random, what is the chance of getting a yellow pair?

[teaser2930]