The figure shows a quadrilateral ABCD with
angles BAD, BAC, BCA, and ACD equal to 90, 22.5, 30, and 15
degrees, respectively, Find the measure of angle ABD.
This entry contributed by Abdilkadir ALTINTAS and Ayhan YANAGLIBAS).
When asked how he developed his mathematical abilities so rapidly, he replied "by studying the masters, not their pupils." Abel, Niels Henrik. 1802-1829.