$13.43
Given: To mail a first-class letter, it costs $0.13 for one ounce. From then, the additional ounce's cost $0.7.
Hence, equation can be created:
y = 0.13 + 0.7(x) where 'y' is total cost, 'x' is additional ounces
Here,
additional ounces: 19.89 - 1 = 18.89 ounces [tex]\sf \underset{}{ \xrightarrow{ \text{ rounded to nearest ounce} } }[/tex] 19 ounces
Total cost: y = 0.13 + 0.7(19) ⇒ y = 13.43
