Let you go x miles daily.
In option A, you have to pay ($25 + 15 cents per mile)
cost = $25 + $(15×x)/100 = $(25+0.15x)
In option B, you have to pay ($10 + 40 cents per mile)
cost = $25 + $(40×x)/100 = $(10+0.40x)
In both options, equal amount will be paid when
25 + 0.15x = 10 + 0.4x
⇒ 25 - 10 = 0.4x - 0.15x
⇒ 15 = 0.25x
⇒ 0.25x = 15
⇒ x = 15/0.25 = 60
So If I travel 60 miles, then i have to pay equal amounts in both plans. Initially, I have to pay more in option A, so if i travel more than 60 miles a day, then I will have to pay less and A will be cheaper.
So for more than 60 miles daily, option A is cheaper.