GraphQL: How to structure mutation to add an item to a list of ints?

Question

I am using graph.cool and Apollo. This is a simplified schema below. I want to add an int to the list and compute a new average.

GraphQL Schema for a Test model. How would I add another int to the list property?

type Test @model {
  id: ID! @isUnique
  list: [Int!]
  average: Int
}