Why this example prints 17?

Question

Good evening coders. I started to learn java and I finaly done with arrays, but i dont understand why following example returns 17.

https://code.sololearn.com/cwITzSH1f4hG/?ref=app

Answer 1

In the loop there an addition inside conditional operator
0+1+2+10+4 = 17

Answer 2

It is compound assignment operation
a +=1 is the same as a = a + 1
You could use
+= -= *= /= %=

Answer 3

Thanks, but what does a "+=" symbol? I didn't get it at all.