The ratio of length to width is 3/2. Let's set up equations of ratios using first the 25 cm length of the paper and then the 20 cm width:
3 25 cm
--- = ----------
2 x
Solving for x, 3x = 50 cm, and x =16 2/3 cm. This is possible, since 16 2/3 is less than the paper width 20 cm.
3 20 cm
--- = -----------
2 x
Solving for x: 3x = 40 cm; x = 40/3 cm, or x= 13 1/3 cm. This is possible, but does not make maximum use of the 20 by 30 cm paper.
Answer: the largest flag Jake can draw on the paper given is 20 cm (length) by 16 2/3 cm.
