Answer:
Part A: The total cost of 10 stamps  =  $3.4
Part B: The total cost of 100 stamps  =  $34
Step-by-step explanation:
The cost of each postcard stamp = $0.34
Part A:
Number of postcard Natalie buys  =  10
So, the cost of 10 postcard stamps  =  10 x Cost of each stamp
                               = 10  x ($0.34)  = $3.4
or, the total cost of 10 stamps  =  $3.4
Part B:
Number of postcard Natalie buys  =  100
So, the cost of 100 postcard stamps  =  100 x Cost of each stamp
                               = 100  x ($0.34)  = $34
or, the total cost of 100 stamps  =  $34