請教obj-c高手們

#import <Foundation/Foundation.h>

@interface Calcuator : NSObject

-(void) setAccumulator: (double) value;
-(void) clear;
-(double) accumulator;

-(void) add: (double) value;
-(void) subtract: (double) valur;
-(void) multiply: (double) valur;
-(void) divide: (double) valur;

@end

@implementation Calcuator
{
double accumulatoir;
}

-(void) setAccumulator:(double)value
{
accumulatoir = value;
}

-(void) clear
{
accumulatoir = 0;
}

-(double) accumulator
{
return accumulatoir;
}

-(void) add:(double)value
{
accumulatoir += value;
}

-(void) subtract:(double)valur
{
accumulatoir -= valur;
}

-(void) multiply:(double)valur
{
accumulatoir *= valur;
}

-(void) divide:(double)valur
{
accumulatoir /= valur;
}
@end

int main(int argc, const char * argv[])
{

@autoreleasepool {
Calcuator *deskCalc = [Calcuator new];

[deskCalc setAccumulator: 100.0];
[deskCalc add: 200.];
[deskCalc divide: 15.0];
[deskCalc subtract: 10.0];
[deskCalc multiply: 5];

NSLog(@"The result is %g", [deskCalc accumulator]);


}
return 0;
}


答案 The result is 50（不懂！！）



這是我照教學書上打的～ 剛學一陣子

高手們能否教學一下，看不太懂 ＝＝