Let n represent the date and f(n) represent the money on a specific day.
We see that the pattern is every day, we add $0.50 into our little piggy bank.
So on day 1, we get $0.50 and on day 2, we get $1.00. We can see it is a linear function and since in is increasing by $0.50, our slope m is $0.50.
So we get:
f(n) = 0.5n
Let's test this. On day 3 we get:
f(3) = 0.5(3) = 1.50
So it works! So our answer is:
f(n) = 0.5n
Hopes this helps!