A NY insured carries 100/300 BI and 100/300 SUM. He is struck and seriously injured by a NY tortfeasor whose only policy is the compulsory 25/50/10. His covered damages exceed $200,000. After exhausting the tortfeasor's BI, how much SUM is available from his own carrier in addition to the tortfeasor's payment?

Why this is the answer

New York's SUM endorsement (§ 3420(f)(2) and the NY 28 endorsement) uses a 'limits-offset' or 'reducing' approach: the carrier pays SUM only to the extent the SUM per-person limit exceeds the tortfeasor's per-person BI limit available to the insured. With 100/300 SUM and a 25/50 tortfeasor, the per-person math is 100,000 − 25,000 = 75,000 in additional SUM. The insured collects $25,000 from the tortfeasor and $75,000 from his own SUM carrier, for a total of $100,000 — the SUM limit. This contrasts with 'excess' UIM systems (used in some other states) where the full SUM limit would be paid on top of the tortfeasor's BI. Option B confuses SUM with NY's mandatory UM minimum; D ignores that SUM specifically addresses underinsured tortfeasors.