$12+24=36$ 
The length of Amy\textquotesingle s string is $$12$$ cm. The length of David\textquotesingle s string is $$24$$ cm. What is the total length of their strings? 
Options:
a. $$16$$ cm 
b. $$26$$ cm 
c. $$36$$ cm 
d. $$46$$ cm 