Eight of the digits from 0 to 9 inclusive are used to fill the cells of the crossnumber. What is the sum of the two digits which are not used?
A 12
B 13
C 14
D 15
E 16
10. D There is only one two-digit power of 6 , namely 36 , so 1 down is 36 . Also, there is only one four-digit power of 5 , namely 3125 . Therefore 1 across is 3125 . There are two four-digit powers of 4 , namely 1024 and 4096. However, we know from 1 down that the units digit of 2 across is 6 , so 2 across is 4096 . Hence the only two digits which do not appear in the completed crossnumber are 7 and 8 . Their sum is 15 .