A few things you're given:
b=book
p=pen
c=calculator
b + p + c = 16
Relationships in order to only use one variable:
2p=b
2p+1=c
Equation:
2p + p + 2p+1 = 16
Combining Like Terms/Simplifying and Solving:
5p + 1 = 16
5p = 15
p = 3
Remember the relationship between pen and book.
2p = b so...
2 (3) = $6.00! :)