merge.fsx

type Project = { Number : int; Cost : float }

let projects1 =
    [
        { Number = 1; Cost = 22.50 };
        { Number = 2; Cost = 10.50 }
    ]

let projects2 =
    [
        { Number = 1; Cost = 80.0 };
        { Number = 3; Cost = 100.0 }
    ]

let containsProject projectList project =
    projectList |> List.exists (fun item -> item.Number = project.Number)

let updateProjects projectList project =
    projectList
    |> List.map (fun item -> 
        if item.Number = project.Number then { item with Cost = project.Cost + item.Cost}
        else item)

let merge a b =
    (a, b)
    ||> List.fold (fun state input -> 
        if containsProject state input then updateProjects state input
        else input :: state)

merge projects1 projects2