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because at line 8, your parameter number is equal to 3 and the program calls again the method sum, so it would be 3 + sum(3 - 1) right?
then that where the line 9 appears, at line 9, your parameter...

the sum is: 10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 so it is 55.
how?
first you are going to call that method somewhere in your code.
let say sum(10);.
are you familiar with stack in programming?...