Bernoulli?

You calculate the likelihood that from 30 exactly two, but any onions do not rise. And another two (at the beginning or at the end of the chain) also do not rise. It was about 32 onions.

But it is to be counted from 30 onions only those that lie directly next to each other. The number of possible events is not (30 over 2), but only 29 pairs.

As far as Bernoulli is concerned, understanding of mathematics is more important than applying formulas. The probability that under 30 onions the first two in the row of plants do not rise (here p = 0.15)

P1 = p^2 * (1-p)^28

However, the likelihood P1 also applies to the case that the two onions are at the end of the row or are exactly on rows 5 and 21.

If one asks for the probability that two arbitrary onions do not occur, then there are (30 over 2) = 435 cases. In other words, the two onions can be distributed in 435 permutations in the plant series. This probability is then

P2 = (30 over 2) * P1 = 435 * P1

Strictly speaking, P1 is in total 435 times.

If you ask for the probability that two onions do not rise in pairs, then there are only 29 cases. This probability is then

P3 = 29 * P1

Strictly speaking, P1 is in total 29 times.