I tried to solve the problem
A ten year comparison between the United States and the Soviet Union in terms of crop yields per acre revealed that when only planted acreage is compared, Soviet yields were equal to 68 percent of United States yields. When total agricultural acreage (planted acreage plus fallow acreage) is compared, however, Soviet yield was 114 percent of US yield. From the information above, show that a higher percentage of total agricultural acreage was fallow in United States than in the Soviet Union.
One answer was
If, in a country, there are $x$ fallow acres for every planted acre, yield per planted acre is $1+x$ times the yield per total acre.
Thus the ratio of yields per planted acre between the Soviet Union (S) and the U.S. (U), $.68$, is $1+x_S\over1+x_U$ times the ratio of yields per total acre, $1.14$. Therefore ${1+x_S\over1+x_U}< 1$, whence $x_S<x_U$. This means that in the U.S., there are more fallow acres per planted acre than there are in the Soviet Union, so the percentage of arable land left fallow is higher in the U. S.
What quantity does $\frac{1+x_s}{1+x_u}$ represent in reality?
$1+x_S$ is the ratio of total acres to planted acres in the Soviet Union (How old is your question book?)
$1+x_U$ is the ratio of total acres to planted acres in the United States
So in this sense $\frac{1+x_S}{1+x_U}$ is the ratio between the two countries of the ratio of total acres to planted acres
It then makes some sort of sense to multiply this ratios of ratios by the ratio of yields per total acre to get the ratio of yields per planted acre between the two countries