Below is a cash flow projection on a sequential-pay CMO, which has four tranches, A through D. Assume the four tranches get paid in the order of A->B->C->D. In this month tranche A has an outstanding balance of 86,000 and B has an outstanding balance of 31,000. What is the principal payment towards tranche B in this month?

month	beginning balance	scheduled principal payment	Prepayment	Interest to MBS investors
20	$631,000	30,000	13,000	46,000