\(\text{In general an arithmetic sequence has the explicit form }\\ s_n = n d + s_0\\ \text{It's recursive form is }\\ s_n = s_{n-1}+d\\ \text{In both cases }s_0 \text{ is specified}\)
\(fine_5=2.25=5d+fine_0\\ fine_{21}=6.25=21d+fine_0\\ fine_{21}-fine_5 = 4= 16d\\ d=\dfrac 1 4\\ 2.25=\dfrac 9 4 = \dfrac 5 4 + fine_0\\ fine_0=1\)
\(a) ~fine_n = fine_{n-1}+0.25,~fine_0=1\\ b)~fine_n = (0.25)n + 1\)
\(c) ~\text{Hopefully clearly he'll choose the explicit formula. Think about it.}\)
\(d)~fine_{60}=60(0.25)+1 = 16\)
.