The author was a celebrity in the second half of the 20th century for his Mathematical Problem Collections for middle school students.
Ironically, his name has come up in a controversy. See here and here.
I solved it from his 1973 collection. The 1975 edition, slightly modified, is here. A list of exercises is selected here and here.
I liked the following problem which seems like Elementary Arithmetic :
"Un obiect se vinde cu 39 lei, castigandu-se atat la suta cat a costat
obiectul. Care a fost costul obiectului ?"
In translation :
"An object is sold for 39 lei, earning a percentage of the cost of the object.
What was the cost of the object ?"
The answer is 30 lei. That is, an object that costs 30 lei was sold for 30% more. That is, a commercial addition of $30\cdot \frac{30}{100}=9$ lei. So the selling price is
30 lei + 9 lei = 39 lei.
In my personal edition the problem appears on page 136, Problem #5. In the 1975 edition the problem appears on page 201, Problem #5. Among more modern editions, the problem appears on page 192, Problem #22.
How can this problem be solved arithmetically ?
I don't know the answer, but I solved it using algebra. In fact, the problem is included in the Chapter on 2nd Grade Equations.
Solution CiP
Let $x$ be the initial price of the object. The object is being sold for $x\text{%}$ more. The commercial markup is therefore $x\cdot \frac{x}{100}=\frac{x^2}{100}$. The selling price of the object will be $x+\frac{x^2}{100}$.
So we have the quadratic equation
$x+\frac{x^2}{100}=39$
In real numbers the equation has two solutions $x_1=30,\;\;x_2=-130$. Of these, only the first has significance for our problem.
$\blacksquare$
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