MadeEasy Test Series: Algorithms - Time Complexity

Question

Can someone prove this?

Comments

i : 1    2    3    4    5    6 ....

j : {1}  {1234} {123456789}  ........

out of which exactly i elements satisfy % condition

For i=2 and j={1234}, 2 elements satisfy the condition.

for j=2 , k loop runs 2 times , for j=4 ,k loop runs 4 times ,

For j=n, k loop runs n times but out of $n^2$ elements of j we extracted n =>O($n^3$).

So,outer loop runs n times  =>Total = O($n^4$).