Help me understand this Strange C++ code

Question

This was a question in our old C++ exam. This code is driving me crazy, could anyone explain what it does and - especially - why?

int arr[3]={10,20,30};
int *arrp = new int;

(*(arr+1)+=3)+=5;
(arrp=&arr[0])++;

std::cout<<*arrp;

Answer 1

This statement writes to the object *(arr+1) twice without an intervening sequence point so has undefined behavior.

(*(arr+1)+=3)+=5;

This statement writes to the object arrp twice without an intervening sequence point so has undefined behavior.

(arrp=&arr[0])++;

The code could result in anything happening.

Reference: ISO/IEC 14882:2003 5 [expr]/4: "Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression."

Answer 2

(*(arr+1)+=3)+=5;

arr + 1 - element with index 1
*(arr + 1) - value of this element
(arr + 1) += 3 - increase by 3
((arr+1)+=3)+=5 - increase by 5;

so arr[1] == 28

(arrp=&arr[0])++;

arr[0] - value of element 0
&arr[0] - address of element 0
arrp=&arr[0] - setting arrp to point to elem 0
(arrp=&arr[0])++ - set arr to point to elem 1

result: 28