when I enter this
INSERT INTO works_on
(essn, pno, hours)
values
(\'123456789\', 1, 32.5),
(\'123456789\', 2, 7.5),
(\'666884444\', 3, 40.0),
(\'453453453\',
You cannot combine all your values in a single insert like that in Oracle unfortunately. You can either separate your SQL statements, or use another approach like this to run in a single statement:
INSERT INTO works_on (essn, pno, hours)
SELECT '123456789', 1, 32.5 FROM DUAL UNION
SELECT '123456789', 2, 7.5 FROM DUAL UNION
SELECT '666884444', 3, 40.0 FROM DUAL UNION
SELECT '453453453', 1, 20.0 FROM DUAL UNION
...
SELECT '987987987', 30, 5.0 FROM DUAL UNION
SELECT '987654321', 30, 20.0 FROM DUAL UNION
SELECT '987654321', 20, 15.0 FROM DUAL UNION
SELECT '888665555', 20, 0 FROM DUAL
SQL Fiddle Demo