Performing arithmetic on double

I have an app whose purpose is to compare chronologically ordered time intervals, which are stored by Core Data (via MagicalRecord) as attributes of type double, on an entity called TimedActivity. Ordering directions are supplied by attributes called activityOfInterest and benchmarkActivity on another entity named SearchSpecs. The scheme may seem a bit overcomplicated since I'm pretty green, but that part of it works.

The problem is that getting percentages from two doubles appears to be a bit of a runaround, at least according to the research I've done. I don't need extreme precision. Round seconds are fine. I found a suggestion relating to the use of NSDecimalNumber, but it too seemed like a long way around the corner.

Here is the relevant code in it's current state, with pseudocode to indicate my problem area:

#pragma mark - Go button case handlers

-(void) handleAvsAAction
{
//    NSArray *searchSpecsObjects = [SearchSpecs MR_findAll];
//    NSLog(@"SearchSpecs number of objects is %i",[searchSpecsObjects count]);

    NSArray *searchSpecsArray = [SearchSpecs MR_findAll];
    NSLog(@"searchSpecsArray count is %i", [searchSpecsArray count]);

    SearchSpecs *thisSpec = [searchSpecsArray objectAtIndex:0];
    NSLog(@"SearchSpec activityOfInterest should be %@", thisSpec.activityOfInterest);
    NSLog(@"SearchSpec benchmarkActivity should be %@", thisSpec.benchmarkActivity);

    //    NSArray *activityOfInterestArray;
//    NSArray *benchmarkActivityArray;
    NSNumber *activityOfInterestDurationTotal;
    NSNumber *benchmarkActivityDurationTotal;

    NSPredicate *activityOfInterestPredicate = [NSPredicate predicateWithFormat:@"name == '%@'",thisSpec.activityOfInterest];

    NSPredicate *benchmarkActivityPredicate = [NSPredicate predicateWithFormat:@"name == '%@'", thisSpec.benchmarkActivity];

    activityOfInterestDurationTotal = [TimedActivity MR_aggregateOperation:@"sum:" onAttribute:@"duration" withPredicate:activityOfInterestPredicate];
    NSLog(@"The sum of all the durations for the activity of interest is %zd", activityOfInterestDurationTotal);

    benchmarkActivityDurationTotal = [TimedActivity MR_aggregateOperation:@"sum:" onAttribute:@"duration" withPredicate:benchmarkActivityPredicate];
    NSLog(@"The sum of all the durations for the benchmark activity is %zd", benchmarkActivityDurationTotal);

    [self doTheMathAvsA];

}

-(void) doTheMathAvsA
{
    // Get the total and respective percentages of the totalled durations from the criteria distilled in handleAvsAAction

NSNumber *total;
total = (activityOfInterestDurationTotal + benchmarkActivityDurationTotal);

}

Edit: modified doTheMathAvsA to clarify the desired result.

All help or suggestions appreciated!

Second edit:

OK, your answer below makes sense, and thanks @Martin R!

However, the two quantities in question here originate as NSTimeIntervals, and as mentioned above, are stored as attributes of type double, on an entity called TimedActivity.

So, it seemed rational to me to slightly rewrite the code to extract them from the persistent store as NSTimeIntervals, which I am assured are really just doubles. However, when I do this, I get this error:

Assigning to 'NSTimeInterval' (aka 'double') from incompatible type 'NSNumber *'

Here are the modified declarations:

NSTimeInterval activityOfInterestDurationTotal;
NSTimeInterval benchmarkActivityDurationTotal;

And here's where the error appears:

activityOfInterestDurationTotal = [TimedActivity MR_aggregateOperation:@"sum:" onAttribute:@"duration" withPredicate:activityOfInterestPredicate];
NSLog(@"The sum of all the durations for the activity of interest is %@", activityOfInterestDurationTotal);

benchmarkActivityDurationTotal = [TimedActivity MR_aggregateOperation:@"sum:" onAttribute:@"duration" withPredicate:benchmarkActivityPredicate];
NSLog(@"The sum of all the durations for the benchmark activity is %@", benchmarkActivityDurationTotal);

OK, I assume that the NSNumber referred to in the error message is this property in the TimedActivity managed object subclass, auto-generated from the data model:

@property (nonatomic, retain) NSNumber * duration;

So my question becomes:

Is it really necessary to resort to such seemingly ever-widening circles of conversion and retro-conversion to perform such a seemingly simple calculation? Or am I missing a more straightforward solution?

Thanks!