Hello!

I noticed that Your answer goes through way too many iterations and doesn't take large enough steps, there can be no correct answer between a.Max() and 2a.Max(), therefore skipping those is a good idea.

Looping through by generating an amount of integers also isnt really necessary, just start from lowest possible correct answer ( a.Max() ) and make sure youre not over the largest possible ( b.Min() ) correct answer.

Works like a charm :).

Here is the resulting code with comments.

public static int getTotalX(List<int> a, List<int> b)
{
/*Starting from 0 found, storing largest value in A and smallest value in b. Setting starting point to largest value in A.*/
    int foundCount = 0, maxA = a.Max(), minB = b.Min(), current = maxA;
    while (current <= minB)
    {
/*If the current value is divisible by all members of both arrays, then it's the one we want.*/
        if (a.All(e => current % e == 0 || e % current == 0) && b.All(e => current % e == 0 || e % current == 0))
            foundCount++;
/*Iterate the value by largest member of divisors, no reason to take smaller steps.*/
        current+= maxA;
        };
    return foundCount;
}

" C# " for everyone CTRL+F-ing.

//Stored a.Max() and b.Min() as dsgarg71 suggested below :).