A taxi ride costs 2.75 for the first quarter of a mile and 0.85 for each susequent quarter mile

Equation would be

for the nth mile the cost would be

f (n) = (n-1) 0.85+2.75

that way, for the 1st mile, the 0.85 doesn't apply

then for the next miles, it does

we can distribute it

0.85 (n-1) = 0.85n-0.85

so we have

f (n) = 0.85n-0.85+2.75

f (n) = 0.85n+1.9

the new equtation is cost for the nth mile is 0.85n+1.9