Offspinner Saeed Sarkar (3/6) and leftarm spinner Saleh Ahmed Shawon Gazi (3/32) claimed three wickets each to skittle out Afghans for 99 in 26.2 overs after they opted to bat in a must-win match.
Bangladesh then returned to chase down the target in less than 25 overs, losing two wickets.
Bangladesh were 29 for 2 in 7.2 overs, losing two wickets in space of five runs, but skipper Mehedi Hasan Miraz (37 not out from 50 balls; 6x4) and Nazmul Hossain Shanto (38 not out from 60b; 4x4) put on an unbroken 72 at a fairly brisk clip to steer their team home in 24.3 overs and bag a bonus point against Afghanistan who failed to notch any win.
Earlier, Afghanistan were given a steady start by openers Hazratullah (16) and skipper Ihsanullah (12) who put on 34 in 5.4 overs but Bangladesh skipper Mehedi Hasan Miraz (1/19) triggered the collapse with his gentle offspin.
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Leftarm pacer Mehedi Hasan Rana (2/33) joined in the act to reduce them to 34/3 in 6.3 overs by taking two wickets in three deliveries.
The leftarm spin and offspin duo of Saleh Ahmed Shawon Gazi and Saeed Sarkar continued the free flow of wickets with the Afghan batsmen in no mood to build a platform and rather going for their shots as only two batsmen, apart from the openers, managed double-digit scores.
Afghanistan Under-19: 99 all out in 26.2 overs (Tariq 27; Saleh Ahmed Shawon 3/32, Saeed Sarkar 3/6, Mehedi Hasan Miraz 2/33) lost to Bangladesh Under 19: 101/2 in 24.3 overs (Nazmul Hossain Shanto 38 not out, Mehedi Hasan 37 not out; Tariq 1/13).