Problem: Alan Wayne E I N + E I N + E I N + E I N = V I E R Solution: Select the two digits E and I. 4 * (10E + I) + c = 10I + E (mod 100) We are looking for solutions with a carry 0<=c<4. Simplification gives 39*E + c = 6*I (mod 100) Create a table trying all different values of E. If E is fixed the values of I and c can be determined. E 39*E 39*E (mod 100) I 6*I c good of bad? 0 0 0 0 0 0 E = I 1 39 39 7 42 3 good 2 78 78 13 78 0 I > 9 3 117 17 3 18 1 E = I 4 156 56 10 60 4 I > 9, c > 3 5 195 95 16 96 1 I > 9 6 234 34 6 36 2 E = I 7 273 73 13 78 5 I > 9, c > 3 8 312 12 2 12 0 good 9 351 51 9 54 3 E = I Now check the two good candidates. case E=1, I=7, c=3: We get V=0 so we have a leading zero. To get c=3 we must have 4*N >= 30. Therefore we can have N=8 or N=9. This gives the solutions 4 * 178 = 0712 and 4 * 179 = 0716. case E=8, I=2, c=0: We get V=3. To get c=0 we must have 4*N < 10. Therefore we have N = 1, as 2 is used already and 0 would give R=0 too. This gives the solution 4 * 821 = 3284. Reference: Martin Gardner, Scientific American (Dec. 1975), p116