Which is more economical: purchasing the economy size of a detergent at 2 kilograms for $3.15 or purchasing the regular size at 890 grams for 60cents?

Economy size of detergent; 2 kg of detergent cost 3.15=> 1 kg  

Costs (3.15/2)

Since 1 kg= 1000 g and 1$=100 cents we have  

1000 g of detergent costs ( 3.15/2)*100 cents or  

1 g of detergent costs ( 3.15/2)*100/1000

=3.15/20 cents

Regular size detergent 890 g for 60 thus 1 g costs 60/890 cents  

Lets compare 3.15/20 and 60/890

2/43> 3.15/2

The regular size of detergent  is more economical

a² + b²= c²

24² + 7² = c²

c= 25

60%