we know the pond has 700 liters to begin with, before any additional liters are added at the tune of 35 liters per minute, so after 1 minute we can say that the total amount of water is 700 + 35(1).
let's make a quick table of it
minutes........total water
1....................700 + 35(1)
2....................700 + 35(2)
3....................700 + 35(3)
4....................700 + 35(4)
5....................700 + 35(5)
6....................700 + 35(6)
T....................700 + 35(T)
W = 700 + 35T
Check the picture below.