CPR where, practice, single

LorrinCFA

Schweser practice exam vol 1, Exam 1 Afternoon Session

Q109 Determind the single month mortality rate for MBS-X for month 20 using 200PSA.

Solution says step 1) find the appropriate CPR by using 6%*(20/30)*2

but where does 6% and 30(denominator) come from?

Thanks.