Given :
y = -0.1x + 1
where,
y = percentage,
x = time in seconds
Now, substituting the values from the question into the equation, we get,
50% = -0.1x +1
.50 - 1 = -0.1x
-.50 = -0.1x
-.50 / (-0.1) = x
5 = x
x = 5
So, the time taken to download 50% of the song is 5 seconds