You gotta pick number (Miles) that give you higher figure for 0.15 and lower figure 0.10 so that the actual answer comes same after adding them with per day charges.
Let's pick 100 miles for A and B,
A, 0.10x 100 = $10
= ($35 + $10)
=$45
B, 0.15 x 100 = $15
= ($25 + $15)
=$40
Well, it ain't giving us same charge, so we should go higher miles
Let's take 200 miles this time.
A, 0.10x 200 = $20
= ($35 + $20)
=$55
B, 0.15 x 200 = $30
= ($25 + $30)
=$55
Answer: after 200 miles.