CREATE TABLE table_15988 ("Class" TEXT, "Operator" TEXT, "No. Built" FLOAT, "Year Built" TEXT, "Cars per Set" FLOAT, "Unit nos." TEXT)

What year was number 7 built?
SELECT "Year Built" FROM table_15988 WHERE "No. Built" = '7'
SELECT "Year Built" FROM "table_15988" WHERE "No. Built" = '7'