PM,s original bac was 600k, now to finish project he/she need extra 20 k..so total bac will be 620k. Specifying mgt reserve here is only extra information... 

for example I gets one apporoved cc and total cost for it is 20k. Now then we decides from where I will get this money, best way would be sponser approves another 20 k for the project but most of the time ;in real world we deplete mgt reserves. 

Fyip- evm calculations don't consider mgt reserves as they are not part of baseline for cost.

 In addition of Mr. Sandeep's explanation, few points :

this is a question of Original cost baseline and then a revised baseline.

Along with baseline - in running project other 2 lines also run - one is actual expenditure and second is funding line( stepped line)

Management reserve can be used as expenditure and funding line can have mangement reserve amount as always more than the cost base line. Till no revised baseline CV = EV - AC , will work there.

As soon revised baseline is framed new BAC will be there in place of EAC.

In present question key point is project is at completion with 20000 extra amount.

At completion always EAC will be termed as BAC of revised baseline.

please correct me , if there is any thing deviating from the PMBOK concepts.

Formula#1

 Estimated cost + contingency reserve = Cost baseline

Formila#2

Cost baseline + management reserve = Budget at completion

We can use Formula#2

Budget at completion = Cost baseline + management reserve = 600,000 + 20,000 = $620,000 Ans