Solving math problems with model diagrams

Mark bought a pen and a book. The book cost $30. If the book costs 3 times as much as the pen, how much did Mark pay for both items?

    3 units = $30
    1 unit  = $30 ÷ 3   = $10
    4 units = 4 × $10   = $40
    Mark paid $40 for the pen and the book.

Mark bought a pen and a book. The book costs 3 times as much as the pen. If the book costs $30 more than the pen, how much did Mark pay for both items?

    2 units = $30
    1 unit  = $30 ÷ 2   = $15
    4 units = $4 × $15   = $60
    Mark paid $60 for the pen and the book.

2 pens and a book cost $36. The book cost $6 more than a pen. Find the cost of the book.

    3 units + $6 = $36
    3 units = $36 - $6   = $30
    1 unit  = $30 ÷ 3   = $10
    1 unit + $6 = $16
    The book costs $16.