can you please explain to me the concept behind the question
Concept behind question
@Kiraa_545, this is a very classical problem famously known as CHINESE REMAINDER PROBLEM . the Chinese remainder theorem states that if one knows the remainders of the Euclidean division of an integer n by several integers, then one can determine uniquely the remainder of the division of n by the product of these integers, under the condition that the divisors are pairwise coprime.
you can solve this question by using brute force approach in which we start with 1 and one by one increment it and check if dividing it with given elements in num[] produces corresponding remainders in rem[]. Once we find such an x, we return it.
but this approach is quite inefficient in practice
fortunately for us , there is a formula stated in chinese remainder theorum ,
if you don’t know about it i highly suggest you to look at it first.
RESOURCES ------------------>>
If you have access to the vedio through your course then refer that else there is a youtube vedio (check below) you can refer that
start from time stamp : 1:19:31
for theory ref :-
if you want to look at bruteforce approach ref:-
In case of any doubt feel free to ask 
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