3)let cost of 1 kg of apples=RS.x
Cost of 1 kg grapes=rs.y
A.t.1st.c,
2x y =160----(1)
x=80.  y=0
x =60.  y=40
A.t.2nd.c 
4x 2y =300
2(2x y)=300
2x y =150

x=75   y =0 
x =50. y =50

2) let cost of 1 bat =rs.x
Cost of 1 ball = RS.y 
A.t.1st.c 
3x 6y =3900
3(x 2y)= 3900
x 2y = 1300---(1)
If x=0  y=650
x = 300  y=500

A.t.2nd.c
x 3y = 1300----(2) 
x=400. y = 300
x = 100.   y = 400

Let present age of Aftab=x
Present age of daughter= y
7yrs ago,
Age of Aftab= x-7
Age of daughter =y-7
A.T.1st.C,
(x-7)= 7(y-7)
x-7 = 7y -49
x-7y = -49 7
x -7y = -42------(1)

If we put x= -7 then y become 5
If we put x=0 then y become 6
3yrs later,
Age of Aftab (x 3)yrs
Age of daughter=(y 3)
A.T.2nd.C
x 3=3(y 3)
x 3 = 3y 9
x-3y = 9-3
x-3y = 6-----(2)
If we put x= 0 then y become-2
If we put x=6 then y become=0