Firstly, solve 38C15.
Then solve 42C12.
Compare your answers. 
The goal here is to see if 38C15 = 42C12.
Here is the formula:
nCr = n!/(r!(n - r)!
Go for it. 
 
        
             
        
        
        
Answer:
   -1 -3i
Step-by-step explanation:
It is unusual to have two imaginary terms grouped together like this. Taking the question at face value, we treat i as though it were a variable, and collect terms in the usual way.
   (-2i +3i) +(-1 -4i) = i(-2+3-4) -1 = -1 -3i
 
        
             
        
        
        
Nine tenths, four hundredths, and one thousandth.
OR:
Nine hundred and forty one thousandths.
        
                    
             
        
        
        
Step-by-step explanation:
1 km = 1000 m
(hence the term "kilo" which means 1000).
so. he runs 1000 m at the rate of 3 minutes per km, that means 3 minutes per 1000 m.
now, we can see the solution already with our eyes.
but let's do it formally :
the rate or ratio between time and distance must be the same for the same speed. 
so,
3 min / 1000 m = x min / 500 m
500 m × 3 min / 1000 m = x min
1/2 × 3 min = x min
3/2 = 1.5 min = x min
so, it takes him 1.5 minutes (90 seconds) to run 500 m.