Add Running Total for each Item

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content], 
    data = List.Buffer(Table.ToRows(Source)), 
    gen = List.Generate(
        () => [i = 0, item = data{0}{0}, total = data{0}{2}],
        (x) => x[i] < List.Count(data), 
        (x) => [i = x[i] + 1, item = data{i}{0}, total = data{i}{2} + x[total] * Number.From(item = x[item])],
        (x) => data{x[i]} & {x[total]}
    ), 
    tbl = Table.FromRows(gen, Table.ColumnNames(Source) & {"TOTAL TIME"})
in
    tbl

Hi Emily_Kim  Go to power query and add a index column. See image below:

Go to home tab and click advance edition and add the below marked line. Using List.Sum and List.FirstN function you can acheive your desired result. See image below:

You will get your desired output. See image below:

Now you can remove index column and change type of running total.

Hope this helps!!

If this solved your problem, please accept it as a solution!!

Best Regards,
Shahariar Hafiz

Check this one step code:

let
    Source = Excel.CurrentWorkbook(){[Name="Table5"]}[Content],
    X = Table.AddIndexColumn(Source, "Index", 1, 1, Int64.Type),
    #"Added Custom" = Table.AddColumn(X, "Custom", each List.Sum(Table.SelectRows(X,  (a)=> a[Index]<=[Index] and a[Item]=[Item])[TIME]))
in
    #"Added Custom"