Video narration
Boat A-vin downstream speed evvalavu endru kandupidikka mudiyuma? Mudhalil thevaiyana formula-kalai paarppom: upstream speed = still-water speed - stream speed. Downstream speed = still-water speed + stream speed. Upstream endraal river current-kku ethiraaga boat selvathu. Downstream endraal current sellum thisaiyileye boat selvathu. Bicycle-ai kaatrukku ethiraaga ottinaal speed kuraiyum; kaatrudan serthu ottinaal speed koodum allavaa? Boat-kkum ithe logic thaan. Ippodhu kodukkappatta distance matrum time-ai vaiththu actual speeds-ai kandupidippom. Boat A, upstream-aaga 90 km thoorathai 5 hours-il selgiradhu. Aagave, Boat A-vin upstream speed = distance divide time = 90 divide 5 = 18 km/h. Boat B, downstream-aaga 180 km thoorathai 5 hours-il selgiradhu. Enave, Boat B-vin downstream speed = 180 divide 5 = 36 km/h. Ippodhu mukkiyamaana ratio: still-water-il Boat A matrum Boat B-vin speed ratio 4:5. Ratio-vai actual speed endru ninaikkakoodaadhu. Athanaal, Boat A-vin still-water speed-ai 4x endrum, Boat B-vin still-water speed-ai 5x endrum eduththukkolvom. Ore river enbadhaal, irandu boats-kkum stream speed ondruthaan. Athai s endru ezhuthalaam. Ippodhu formula-vai payanpaduththinaal, Boat A upstream: 4x - s = 18. Boat B downstream: 5x + s = 36. Inge oru sulabamaana trick irukkiradhu. Irandu equations-ai add seithaal, -s matrum +s cancel aagividum. Aagave, 4x + 5x = 18 + 36. Athanaal 9x = 54, enave x = 6. Athanaal Boat A-vin still-water speed = 4x = 4 x 6 = 24 km/h. Mudhal equation-il substitute seithaal, 24 - s = 18. Enave stream speed s = 6 km/h. Ippodhu kelvikkana irudhi padi: Boat A downstream-aaga sellumbodhu current speed serum. Aagave downstream speed = 24 + 6 = 30 km/h. Enave, Boat A-vin downstream speed 30 km/h.