#### Answer

It is more economical to purchase the regular size.

#### Work Step by Step

We can find the cost per kg for the economy size.
$Cost = \frac{price}{weight}$
$Cost = \frac{\$3.15}{3~kg}$
$Cost = \$1.05~per~kg$
We can convert 720 g to units of kg. We know that 1 kg = 1000 g, so we need to move the decimal point 3 places to the left.
720 g = 0.72 kg
We can find the cost per kg for the regular size.
$Cost = \frac{price}{weight}$
$Cost = \frac{\$0.60}{0.72~kg}$
$Cost = \$0.83~per~kg$
Since the cost per kilogram for the regular size is less than the cost per kilogram for the economy size, it is more economical to purchase the regular size.