Oracle SQL Complications With Multiple Triggers

Hey all so I am creating a DB that is used going to be used for managing utilities of a system. My DB has 3 tables: Users, Utilites, and Files (leaving Files table out because question shouldn't affect it).

TABLES

CREATE TABLE USERS (
User_id Number(38,0) NOT NULL PRIMARY KEY,
User_name char(18) NULL ,
User_pass varchar(18) NULL ,
Storage_Size varchar(18) NULL ,
Memory_Usage Number(38,0) NULL
);

CREATE TABLE UTILITIES (
Utility_id Number(38,0) NOT NULL PRIMARY KEY,
Utility_Name varchar(18) NULL ,
Cost Number(38,0) NULL ,
Running char(4) NULL ,
User_id Number(38,0) NULL ,
);

TRIGGERS

//Trigger to SUM the cost of all UTILITIES:

CREATE OR REPLACE TRIGGER memory_after_insert
AFTER INSERT OR UPDATE OR DELETE
ON UTILITIES
BEGIN
UPDATE USERS
SET MEMORY_USAGE = (SELECT SUM(COST) FROM UTILITIES WHERE USERS.USER_ID = UTILITIES.USER_ID);
END;

//Trigger to SUM the cost of all FILES

CREATE OR REPLACE TRIGGER storage_after_insert
AFTER INSERT OR UPDATE OR DELETE
ON FILES
BEGIN
UPDATE USERS
SET STORAGE_USAGE = (SELECT SUM(FILE_SIZE) FROM FILES WHERE USERS.USER_ID = FILES.USER_ID);
END;

Now I would like to create a Trigger that on INSERT into USERS (User creation) will INSERT into UTILITIES all of the systems Utilities (I have X default utilities I would like every user to have)

Example Trigger I have come up with:

CREATE OR REPLACE TRIGGER users_after_insert
  AFTER INSERT ON USERS
  FOR EACH ROW
BEGIN
  INSERT INTO UTILITIES (UTILITY_NAME, RUNNING, USER_ID, UTILITY_ID, COST)
  VALUES
  ('Javaw.exe', 'YES', :new.USER_ID, seq_utility_id.nextval, round(dbms_random.value(25000, 100000)));
  INSERT INTO UTILITIES (UTILITY_NAME, RUNNING, USER_ID, UTILITY_ID, COST)
  VALUES
  ('Firefox.exe', 'YES', :new.USER_ID, seq_utility_id.nextval, round(dbms_random.value(60000, 200000)));
END;

When I have this trigger in my DB it causes my other triggers to mess up. I get errors: ORA-04091: table STUDENT052.USERS is mutating, trigger/function may not see it
ORA-06512: at "STUDENT052.MEMORY_AFTER_INSERT", line 2
ORA-04088: error during execution of trigger 'STUDENT052.MEMORY_AFTER_INSERT'
ORA-06512: at "STUDENT052.USERS_AFTER_INSERT", line 2
ORA-04088: error during execution of trigger 'STUDENT052.USERS_AFTER_INSERT'

Before adding this third trigger, the other 2 triggers were working perfectly. I assume the problem lies within this trigger. If anyone has any knowledge on how to fix this it would be greatly appreciated! Thanks!