To find the total number of seconds the playlist will play, we first need to calculate the total time in minutes and then convert that to seconds.
First, calculate the total time in minutes:
\[ \text{Total time in minutes} = \text{number of songs} \times \text{average time per song} \] \[ = 1,900 \text{ songs} \times 3.5 \text{ minutes/song} = 6,650 \text{ minutes} \]
Next, convert minutes to seconds. Since there are 60 seconds in a minute, we multiply the total minutes by 60:
\[ \text{Total time in seconds} = 6,650 \text{ minutes} \times 60 \text{ seconds/minute} = 399,000 \text{ seconds} \]
Now, we convert \(399,000\) to scientific notation:
\[ 399,000 = 3.99 \times 10^5 \]
Therefore, the total number of seconds the playlist will play is:
\[ \mathbf{3.99 \times 10^5 \text{ seconds}} \]