I think the question is somewhat ambiguous, but more importantly, the answer provided is not correct. Let me explain.

First I believe the question to be ambiguous. It is ambigous because it does not make clear what is meant by "investment returns" and "Pension Cost". To be clear: In what follows, by "investment return" I will mean the best guesestimate of anticipated return when a pension actuary is estimating the cost of a (given) pension plan. So, for example, currently the best estimate of return on (relatively) safe investments, say, AAA rated bonds of 10 year maturities is around 4 to 5%, whereas during 1980, a comparable return would be close to 8 to 9%.

Next, we have to define what is meant by "Pension Cost": To make the discussion simple, let's go with the example given in the answer, and assume we want to calculate the 4-year university cost of a newly born in 2014. We shall further assume that the newly born will go to university when he/she turns 19, in 2033.

"Pension Cost" can be defined in many ways, but, one actarially reasonable definition would be to say the (equal) monthly amount that (the parent) should set aside so that when the child in 2033 starts college will have sufficient fund to get four year college education.

Using our best estimate of college education inflation, we first estimate the amount of fund that the child will need in 2033 when he/she starts college. Let's call this number $X. Clearly $X is not (at least directly) related to "investment rate of return". For the sake of argument, let's say the amount of college fund will need to be $350,000.

So the question becomes the following: Given interest rate r, what should be the monthly amount, M, that we should set aside in order to have $350,000 in 2033. The answer: if we assume r to be 4.00% (0.33% per month), the monthly outlay, M = $954.41, whereas if we assume r to be, say, 8.00%, the monthly outlay becomes =$594.21.

So I do not know why the person providing the answer has said that the cost will be the same.