Question 565114
Puzzling. I can make a few guesses, but they may all be wrong.
Unless that K was misprinted or misread, there would be an infinite number of answers. Even if we say that K and the price of lunch are a whole number of dollars, there is still an infinite number of answers. Just finding two different possibilities proves that there is not enough information to solve the problem. Janet could have spent $21 on lunch and K would be $5. Janet could be very rich, and could have spent $100 or more on lunch, making K=$400 or more.
If this is grade 4-7, and the problem is exactly as posted, the expected answer may be "there is not enough information." My local school started giving that kind of problem to 4th graders in 1991. They were also supposed to figure out, and state it, if there was superfluous data, not needed to solve the problem.
If this is algebra 2, they may expect the answer as a function:
{{{x=(100+K)/5}}} or equivalent forms {{{x=20+K/5}}} or {{{x=20+(1/5)K}}}
They may expect students to say that x is a linear function of K. With integer values for K, that would be an arithmetic sequence. 
WARNING:
If you are typing an answer in one line x=(100+K)/5={{{(100+K)/5}}} , but
x=100+k/5={{{100+k/5}}} would be incorrect