3. Observe the following number table (horizontal rows are lines):
$$
\begin{array}{l}
\frac{1}{1} ; \\
\frac{2}{1} ; \frac{1}{2} ; \\
\frac{3}{1} ; \frac{2}{2} ; \frac{1}{3} ; \\
\frac{4}{1} ; \frac{3}{2} ; \frac{2}{3} ; \frac{1}{4} ; \\
\frac{5}{1} ; \frac{4}{2} ; \frac{3}{3} ; \frac{2}{4} ; \frac{1}{5} ; \\
. . . . . .
\end{array}
$$

According to the pattern expressed by the first 5 rows of numbers, explain in which row from top to bottom the number $\frac{1991}{1949}$ is located? In this row, where is it located from left to right?
3. 【Solution】First, we notice that the sum of the numerator and denominator of each number in the first row equals 2, the sum of the numerator and denominator of each number in the second row equals 3, ..., and the sum of the numerator and denominator of each number in the fifth row equals 6. From this, we can see a pattern: the sum of the numerator and denominator of each number in a row equals the row number plus 1.

Second, it is clear that the denominator of the first number in each row is 1, the denominator of the second number is 2, ..., i.e., the denominator of the number from left to right is the position of the number.

Therefore, the row number where $\frac{1991}{1949}$ is located equals $1991+1949-1=3939$. In the 3939th row, $\frac{1991}{1949}$ is the 1949th number from left to right.