Question 1048491
s, total start sum of money

Spent equal amount for six days;
x, how much spent each of these six days
r, how much remaining right then, {{{r=s-6x}}}


{{{3/5}}} of s remained after those six days;
MEANING, {{{r=(3/5)s}}}
or
{{{s-6x=(3/5)s}}}



Three more days, he had 360 dollars remaining.
{{{system(s-6x=350, or  (3/5)s=350)}}}
I am not yet understanding your problem description.   At best, you might still have just one equation in two variables.


Did Jason still spend x dollars on each of those three last days?
{{{system(s-6x-3x=350,and,(3/5)s-3x=350)}}}
Now, two equations in the unknown variables x and s.