If payments start NOW (as is specified in problem), then the PV
is calculated using immediate payment formula:
this results in 643.67
If he waits 2 months, then the PV in 2 months will be 637.30;
this needs to be discounted for a further 2 months:
637.30 / 1.01^2 = 624.74
So difference is 643.67 - 624.74 = 18.93
The given answer of 12.68 is not correct: it does not assume
an immediate payment.
It would be correct if 1st payment was not immediate, but in 1 month.