Person A used hot water with soap because hot water helps dissolve soap better and makes it more effective at removing dirt and grease from clothes. Hot water also softens the fibers in the fabric, which can loosen stains and make washing more efficient, especially with natural soaps that may not lather as well in cold water.

Person B used detergent with cold water because detergents are formulated to work well at lower temperatures. Detergents contain chemicals that dissolve quickly in cold water, allowing them to effectively lift dirt and stains without needing hot water. This makes them convenient and energy-saving for cold water washing.