Encourage the students to use systematic counting when calculating the cost of the sea creatures. This will involve looking for common shapes and using multiplication rather than shape-by-shape counting. For example, consider this shape:

Question 3 involves students applying the relationship between the number of trapezia in the body of the sea serpent and the total cost of the creature.
A five-trapezia-bodied serpent costs:

Adding five more trapezia will cost another 75c (5 x 15c), which gives a total cost of $2.10.
This can be organised in a table to check (60c + [number of trapezia x 15c]).

Answers to Activity

1. a. 90c
b. 90c
2. $1.35
3. $2.10
4. Answers will vary.