122. Find the number which, when multiplied by 3, then divided by 5, increased by 6, after which the square root is extracted, one is subtracted, and the result is squared, will give 4.

## Problem of Sridhara.

From the treatise "The Essence of Calculation."
122. The method of inverse actions involves approaching the unknown by performing actions on numbers in the reverse order of those specified in the problem, and, obviously, all actions are replaced by their inverses. Sometimes this technique is called the "method of inversion."

$$
\sqrt{4}=2 ; 2+1=3 ; 3^{2}=9 ; 9-6=3 ; 3 \cdot 5=15 ; \frac{15}{3}=5
$$

![](https://cdn.mathpix.com/cropped/2024_05_21_926f0bb262c8f9569516g-105.jpg?height=57&width=1285&top_left_y=1562&top_left_x=428)
calculations. Bhaskara-Acharya refers to it and even borrows problems from it. The time of his life is not precisely known, but it was certainly in the period between the 6th and 10th centuries AD.