Question

There wear 10000 bugs in a mountain in the train there are 15 bug how many bugs in all

Answer 1

There are 10000 bugs and you add 15 bugs to it. 
10,000+15= 10,015

The total is 10,015 bugs 

There were already 10,000 bugs and 15 bugs joined them. 
Which makes it an addition statement 

Answer 2

So if there's 10,000 bugs in a mountain and 15 bugs in the train, then 10,000 + 15 = 10,015 because in all means to add.