a sequence is a function whose domain is the set of natural numbers. 
natural numbers are all the negative and positive integers
 
        
                    
             
        
        
        
Say x is the number of weeks until their money is equal. Then, we can set up an equation.
400 + 25x = 250 + 75x
Subtract 250 from both sides
150 + 25x = 75x
Subtract 25x from both sides
150 = 50x
Divide by 50 on both sides
3 = x
That means the answer is 3 weeks.
 
        
             
        
        
        
The area of the sector which is the white triangle adding the shaded region is 68.9/360*π*9.28^2=51.78005(rounding to 5th digit after decimal point for accuracy before we do final round for answer)
The area of the white triangle in the sector has area 1/2*9.28^2*sin(68.9)= 40.17223(rounding to 5 digits again for some accuracy.
Now we take out the white triangle from the sector.
51.78005-40.17223=11.60782
rounding to the nearest tenth we get 11.6 cm^2
Problem done!
Hope this helped and if you have any questions about my explanation just ask in the comment and I will answer.