Energy stored in a capacitor of capacitance C with potential differnce V across it is (1/2)CV2.
Let C1 and C2 be the capacitances. V = 600 V.
Equivalent capacitance when they are in parallel is C1 + C2. So, energy stored in this combination is
(1/2)(C
1 + C
2)V
2 = 18 J

(1/2)(C
1 + C
2)(6000 V)
2 = 18 J
Solving, we get C
1 + C
2 = 10
– 6 F = 1

F
Equivalent capacitance of C1 and C2 when they are in series is C1C2 / (C1 + C2).
So, energy stored is (1/2){C1C2 / (C1 + C2)}V2 = 4 J
(1/2){C
1C
2 / (C
1 + C
2)}(6000 V)
2 = 4 J, here put C
1 + C
2 = 10
– 6 F.
Solving we get C
1C
2 = (2/9) x 10
– 12 F
2 = (2/9) (

F)
2 .
Thus, we have C
1 + C
2 = 1

F and C
1C
2 = (2/9) (

F)
2i.e. we want factors of 2/9 whose sum is 1. Such factors are 1/3 and 2/3.