Reduce a 24 cm by 36 cm photo to 3/4 original size.
The most logical way to do this is to keep the width-to-height ratio the same: It is 24/36, or 2/3. The original photo has an area of (24 cm)(36 cm) = 864 cm^2.
Let's reduce that to 3/4 size: Mult. 864 cm^2 by (3/4). Result: 648 cm^2.
We need to find new L and new W such that W/L = 2/3 and WL = 648 cm^2.
From the first equation we get W = 2L/3. Thus, WL = 648 cm^2 = (2L/3)(L).
Solve this last equation for L^2, and then for L:
2L^2/3 = 648, or (2/3)L^2 = 648. Thus, L^2 = (3/2)(648 cm^2) = 972 cm^2.
Taking the sqrt of both sides, L = + 31.18 cm. Then W must be 2/3 of that, or W = 20.78 cm.
Check: is LW = (3/4) of the original 864 cm^2? YES.