Math Stumper

A A C D 9 B C C A 9 C D D D 15 D D A A 10 10 12 11 ? Answer : The answer is 10  d+a+d+a = ? We know that  : a+b+c+d = 10 a+b+c+d = a+a+d+d = 10 a+b+c+c = a+a+c+d = 9 If a+b+c+d = 10 and a+a+c+d = 9 Therefore b-a=1 (or b = a+1) Replace If a+b+c+c=a+a+c+d And b = a+1 Therefore a+a+c+c+1=a+a+c+d Also c+d+1=d+d c+1=d (ou d=c+1) Let's try. We know that a+1= b, and c+1=d and a+b+c+d=10 Therefore 1+2+3+4=10 ? 10=10, yes. a+1=b ? 1+1=2, yes c+1=d? 3+1=4, yes. Therefore : d+a+d+a = 4+1+4+1 = 10