Doubt

Because one mango is left as the question has stated.

31 mangoes divided into group of 2 will be 15 groups and one left. Similarly for 3 and 5.

Once imagine if we have "x" number of mangoes and we make them into groups of 2, groups of 3, groups of 5.

And everytime we do it this way, one mango is left.

So whatever number the "x" is, it isn't divisible by 2,3,5 and also when divided leaves a reminder 1 in each case.

Lemme know if you didn't get, I'll try to explain better.