It is 25% because it is 1/4 which is a quarter.
A must be parallel to b.
Proof:
∠3 = <span>∠13 (given)
</span>∠15 = <span>∠13 (vertically opposite angles are equal)
Since </span>∠13 = ∠3 and ∠13 = ∠15, ∠15 = <span>∠3
Then, a must be parallel to b (transverse line d cuts parallel lines producing converse alternate angles that are equal (ie </span>∠15 = <span>∠3))
</span>
