Table: all_saints_season_10
Columns: no_in_series
no_in_season
title
directed_by
written_by
original_air_date
ratings

Question: What is the sum of the numbers in series written by sam meikle, which have 21 numbers in the season?
SQL: SELECT SUM("no_in_series") FROM all_saints_season_10 WHERE "written_by" = 'sam meikle' AND "no_in_season" = 21