A farmer divides his herd of cows among his four sons so that first son gets one-half of the herd, the second son gets one-fourth, the third son one-fifth and the fourth son 7 cows. The total number of cows in the herd is
(a)100
(b)140
(c)180
(d)240
Answer
Answer (as printed): B
Explanation
Let the total number of cows in the herd be $x$. Then, share of fourth son $=x-\left(\frac{x}{2}+\frac{x}{4}+\frac{x}{5}\right)=x-\frac{19 x}{20}=\frac{x}{20}$. $$\therefore \quad \frac{x}{20}=7 \text { or } x=140 .$$
Explanation as extracted from the printed page; notation may be imperfect.