Since they are rowing against the current, their velocity realtive to the groud is 20-1.5 = 18.5km/h
So the distance travelled is 18.5km/h*0.5h = 9.25km
Going back with the current their ground realative velocity is 20+1.5 = 21.5km/h so the time it takes them to return 9.25 km is 9.25km/(21.5km/h) = 0.43h ~= 26 min
Vaporization - Th<span>e amount of heat needed to convert unit mass of a liquid into vapor form without a change in temperature is called heat of vaporization. For water t</span><span><span>he heat of vaporization is 2260 Joules / gram</span> at its normal boiling point .</span>
