I have a question in java.How and why we find the following output 20 of y?

Question

int arr[]={1,2,3,4,5};
int x=4;
int y=0;
for(int i=0;i<5;i++)
y+=arr[x-i]*i;
System.out.println(y);

Answer 1

When i = 0:
y += 0;
Because arr[4 - 0]*0 ---> 0.

When i = 1:
y += 4;
Because arr[4 - 1]*1 ---> 4*1 ---> 4.

When i = 2:
y += 6;
Because arr[4 - 2]*2 ---> 3*2 ---> 6.

When i = 3:
y += 6;
Because arr[4 - 3]*3 ---> 2*3 ---> 6.

When i = 4:
y += 4;
Because arr[4 - 4]*4 ---> 1*4 ---> 4.

0 + 4 + 6 + 6 + 4 ---> 20.

Answer 2

5*0+4*1+3*2+2*3+1*4=20