Let 'C' be the cost of original diamond and its weight 'w'
C = k.w2 .... (given)
Now, w1 : w2 : w3 : w4 = 1x : 2x : 3x : 4x
w = w1 + w2 + w3 + w4 = 10x
From the given condition,
k.(10x)2 = k.(1x)2 + k.(2x)2 + k.(3x)2 + k.(4x)2 + 70000
k.100x2 = k.30x2 + 70000
k.x2 = 1000
So the original cost of diamond is k.(10x)2 = 100000 = Rs. 1.0 lakh
C is the answer.