How would you go about finding the complexity of this algorithm?

Question

function alg1(n)
1 a=0
2 for o=1 to n do
3     for t=1 to o do
4         for k=t to o+t do
5         a=a+1
6 return(a)

If anyone could guide me to how

Answer 1

Subtract t from the last loop so that it becomes

for k=0 to o do

Now the 2 inner most loops would run for O(o^2) time for every value of o. The answer would be

1^2 + 2^2 + ... n^2

which is equal to

n(n+1)(2n+1)/6. Hence it would be of order of O(n^3)