Answer:
15 minutes
Explanation:
Here the given function that shows the cost( in dollars ) of phone call after taking m minutes,

If the bill is $ 1.90,
That is, C = 1.90
From equation (1),

( Subtracting 0.50 on both sides ),

( Dividing both sides by 0.10 ),
( Adding 1 on both sides ),
Hence, 15 minutes were taken for talking.