There are $50$ numbers. Each number is subtracted from $53$ and the mean of the numbers so obtained is $-3.5$. The mean of the given numbers is

Let the original numbers be $x_1,x_2,\dots ,x_{50}$ and their true mean be $\bar{x}$.According to the question, each element $x_i$ is subtracted from $53$. The new transformed data values are $(53-x_i)$.By linear properties of the arithmetic mean:
$$\text{Mean}(53-x)=53-\text{Mean}(x)$$
We are given that the mean of the new data points is $-3.5$:
$$53-\bar{x}=-3.5$$
$$53+3.5=\bar{x}$$
$$\bar{x}=56.5$$
Thus, the mean of the original group of numbers is $56.5$.