Answer:
A) 

B) Cost function is: 

C) 
 ,
, 

D) 

E) Yes, the result satisfies the equations.
Explanation:
Let number of kilos of mango bought = 

Let number of kilos of atis bought = 

Total number of kilos bought = 20
A) 

Cost of mango per kg = 60
Cost of mango for 
 kg =
 kg = 

Cost of atis per kg = 40
Cost of mango for 
 kg =
 kg = 

Total cost is: Cost of mango + Cost of atis = 1000
B) Cost function is: 

C) System of equations are:
 ...... (1)
 ...... (1)
 ...... (2)
...... (2)
D) Using elimination method:
Multiplying equation (1) with 2 and subtracting from (2):

By equation (1):

E) Putting both the values in LHS of equation (1):
10+10 = 20 = RHS
Putting both the values in LHS of equation (2): 
 = 50 = RHS
 = 50 = RHS
Yes, the result satisfies the equations.