7. A small ink cartridge has enough ink to print 600 pages. Three small cartridges can print as many pages as two medium cartridges. Three medium cartridges can print has many pages as two large cartridges. How many pages can be printed using a large cartridge?
A 1200
B $\quad 1350$
C $\quad 1800$
D 2400
E $\quad 5400$
Solution: B
Three small cartridges have enough ink for $3 \times 600=1800$ pages. So 1800 is the number of pages that two medium cartridges can print. Hence one medium cartridge can print $\frac{1}{2}(1800)=900$ pages. So three medium cartridges have enough ink for $3 \times 900=2700$ pages. So 2700 is the number of pages that two large cartridges can print. Hence one large cartridge can print $\frac{1}{2}(2700)=1350$ pages.

An alternative, algebraic method, is to let $m$ be the number of pages that a medium cartridge can print, and $l$ be the number of pages that a large cartridge can print. From the information we are given we have the equations
$$
3 \times 600=2 m \text { and } 3 m=2 l .
$$

From these we can deduce that
$$
l=\frac{3}{2} m=\frac{3}{2} \times \frac{3 \times 600}{2}=9 \times 150=1350 .
$$