Sorry, I do not know what to call this type of query.

A class of nine kindergarten children is to be taken for a

walk each afternoon. The teacher decides that they will

'walk in sets of three, and any pair of children will walk

together on one day only. For how many days can this

system be maintained without repetition?

(A) 1 (B) 2 (C) 3 (D) 4 (E) 5

I have worked it out by hand and get 4 days

I would like advice on how to approach this question please?

thank you

Cody, Perth