int x=10;
int total=10;
do {
total += x++;
}while(x<15);
System.out.println(x);

The output of this program will be 15. I don't understand why 15 is the answer.

I'm not understanding the arithmetic here. Doesn't the "+=" mean total = total + x? And doesn't x++ mean that the next value for x will be x+1?

I don't see why this would not output 20 on the first go around. 21 on the second go around, etc.

It has little to do with your increment process but with your do-while loop

int x=10;
int total=10;
do {
total += x++;
}while(x<15);
System.out.println(x);

The output of this program will be 15. I don't understand why 15 is the answer.

I'm not understanding the arithmetic here. Doesn't the "+=" mean total = total + x? And doesn't x++ mean that the next value for x will be x+1?

I don't see why this would not output 20 on the first go around. 21 on the second go around, etc.

A rewrite:

int total = 10, x = 10;
do {
  total = total + x;
  x++;
} while(x < 15);
System.out.println("x -> " + x);

Output: x -> 15
int total = 10, x = 10;
do {
  total = total + x;
  x++;
  System.out.println("total -> " + total);
} while(x < 15);

output:
  total -> 20
  total -> 31
  total -> 43
  total -> 56
  total -> 70

Hope you do realize what you are doing wrong here...