I solved two problems in the April long challenge. COVIDLQ and CARSELL.
I got the correct answer in COVIDLQ, and on my first attempt, got the wrong answer in CARSELL. However, in the second attempt for it, I got 30 points and 100 points for the third attempt.
Still, my rating was decreased by 14. Can someone explain to me how the rating mechanism works?
it is all based on your rank for every rated contests held in codechef.
But in case of long challenge wrong submission dosen’t matter.
A person doing a problem in single attempt and another with 100 attempts are treated as same in long challenge.
Ohhhhh ok. So too many users participating in this contest may be the reason why my rating fell, right?
CodeChef follows ELO Rating Mechanism for calculating every user’s rating. You can learn more about it here.
not actually.
But people with lower ratings than you are solving more questions than you can be the reason.
I tried but it seemed a little complicated 
I see… thanks for the clarification!
I know it’s very complicated. BTW, people have shared many rating prediction calculators on Discuss itself. Try using the search function. I hope it will help you. 
elo takes into consideration many factors , a few of which are the number of problems you were expected to solve , your previous rank , total number of competitors ,etc . so for ex , if you solved 2 questions in some long challenge , with 100 participants and you solve 3 questions this time against 1000 participants, dont expect your ratings to change much.