Does 20 – 4x = 12 – x +8 – 3k have infinite solutions, just one or none?

Apatient is to take 60 mg of an antibiotic on day 1, take 45mg on days 2 and 3, take 30mg on days 4 and 5 and take 15 mg on days 6 and 7. how many total mg will the patient take?

Pat listed all the numbers that have 15 as a multiple write the numbers in pats list

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?