title: Getting invalid operand to binary expression ('id' and 'id') and wrong calculation value
body: <p>Getting this error "invalid operand to binary expression ('id' and 'id')" when I try to do some basic math with values stored in an array. The commented out code works, but gives the wrong value for some reason.</p>
  
  <pre><code>@implementation OMOGradesViewController
  
  - (instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
  {
      // Call init method implemented by the superclass
      self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
  
      if(self){
          // Create array of grades
          self.grades = @[@80, @70, @60, @50, @40];
          // self.grades = @[@"80", @"70", @"60", @"50", @"40"];
  
  }
  
      // Return the address to the new object
      return self;
  }
  
  - (IBAction)calculateAvg:(id)sender
  {
  
      for(NSArray *a in self.grades)
          NSLog(@"%@", a);
  
      int *avg = ([self.grades objectAtIndex:0] + [self.grades objectAtIndex:1]);
  
      /*int avg = ((int)self.grades[0] + (int)self.grades[1] + (int)self.grades[2]
      + (int)self.grades[3] + (int)self.grades[4])/5;
  
      NSString *strFromInt = [NSString stringWithFormat:@"%d",avg];
  
      self.averageLabel.text = strFromInt;
      NSLog(@"%@", strFromInt);*/
  
  
  }
  
  @end
  </code></pre>