What problem can be solved by using this multiplication 2/3x3/5

Aquantity with an initial value of 3900 decays exponentially at a rate of 35% every hour. what is the value of the quantity after 0.5 days, to the nearest hundredth?

Solve for x and y: 217x + 131y = 913; 131x + 217y = 827

Seymour is twice as old as cassandra. if 16 is added to cassandra’s age and 16 is subtracted from seymour’s age, their ages become equal. what are their present ages? show !