Jenna's gross monthly income is 20*9*4 + 8*9*1.5 =

720 +108 = 828

Her annual gross is 12*828 = 9936

After 20% taxes, her NET annual salary is 80% of 9936 = 7948.8

After car insurance, she will have 6948.80

Then for her trip to Mexico, she will have 5448.80

DIviding by 12, she has 5448.80/12 = 454.06 per month

to spend on gas and maintenance of her car.