Pebble falls into the pond from the surface and it descends at a rate of 2 feet per second. 
So basically, the pebble is descending or falling down at 2 feet per second which means after every 1 second, it goes 2 feet down. 
The equation to represent the depth of the pebble with time is given by: 

Where, 'd' is the depth after certain time and 't' is the time in seconds. 
Now, we need to find the depth of the pebble after 5 seconds. 
Plugging the value 
, we get: 

So, after 5 seconds the pebble will be at the depth of 10 feet.