A PA28-181 AT 3500 FT; VFR; IS OVERTAKEN BY AN IFR GIV ACFT DSNDING THROUGH THEIR ALT; 450 FT TO THE L. NMAC.

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WORKING WITH STUDENT IN PREPARATION FOR PVT PLT FLT TEST. WAS AT 3500 FT 10 DME WNW BDR VOR; HDG W. HAD MADE CLRING TURNS L AND R; THEN HDG W AT 3500 FT WHEN A GIV PASSED US AT SAME ALT AND 450 FT TO OUR L. GIV WENT FROM OUR 6 O'CLOCK TO OUR 12 O'CLOCK POS. I IMMEDIATELY CONTACTED NEW YORK TRACON ON 126.95 TO SEE IF HE WAS TALKING TO (WORKING) THE GIV. HE (NEW YORK TRACON) SAID HE WAS NOT WORKING THE TFC; BUT SAW HIM AT OUR 12 O'CLOCK POS; 3 MI AHEAD. TFC MUST HAVE BEEN INBOUND TO HPN; AS HE WAS NOT IFR AND NOT TALKING TO APCH FOR A CLASS B CLRNC. I WONDER IF THE CREW OF 2 WAS EVEN LOOKING OR HAD COLLISION AVOIDANCE EQUIP OR WAS JUST TOO LAZY TO GIVE WAY. WE WERE 1200 MODE C. IF WE HAD STARTED OUR NEXT PLANNED MANEUVER (WHICH WAS TO THE L); THERE WOULD HAVE BEEN A COLLISION. OVERTAKING ACFT MUST GIVE WAY. SUPPLEMENTAL INFO FROM ACN 425304: MY ACFT WAS FLYING THE BOUNO 3 ARR INTO HPN. WE WERE IN CONTACT WITH HPN APCH CTL. APCH CTL HAD GIVEN US RADAR VECTORS FOR THE SOUND VISUAL TO RWY 34. WHILE ON VECTORS; WE PASSED UNDER A GA PA28 ACFT FLYING AT APPROX 3500 FT. WE WERE LEVEL AT 3000 FT ON AN ASSIGNED HDG. THE REST OF THE FLT WAS UNEVENTFUL. IT IS MY BELIEF/FEELING THE GA ACFT WAS NOT COMMUNICATING WITH APCH CTL AND NOT AWARE OF BEING WITHIN THE CLASS D AIRSPACE. I BELIEVE THIS COULD BE AVOIDED IF THE OTHER ACFT PLT COULD MAINTAIN PROPER CLRNC FROM CTLED AIRSPACE.