Question

A printer prints 28 photos in 8 minutes. a. How many minutes does it take to print 21 more photos? b. Construct a double number line diagram that represents the situation. How many minutes does it take to print 35 more photos?

Answer 1

now, "21 more photos" is 21 added to the already printed 28, so 21 more is 28+21 photos, or 49,

$\bf \begin{array}{ccll} photos&minutes\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 28&8\\ 49&m \end{array}\implies \cfrac{28}{49}=\cfrac{8}{m}\implies m=\cfrac{49\cdot 8}{28}$

what about 35 more?   well, it already printed 49, and 35 more would then be 84,

$\bf \begin{array}{ccll} photos&minutes\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 28&8\\ 84&m \end{array}\implies \cfrac{28}{84}=\cfrac{8}{m}\implies m=\cfrac{84\cdot 8}{28}$