Let the fixed charge be x and the charge/day be y
\(\therefore\) x+ 20y = 3000 , x + 25y = 3500 ......(1)
\(\Rightarrow\) x = 3000 - 20y
Using x's value in equation(1)
3000 - 20y + 25y = 3500
\(\Rightarrow\) 5y = 500
\(\Rightarrow\) y = 100
\(\therefore\) x = 3000 - 20 \(\times 100\)
\(\Rightarrow\) x = 1000