4.
This is the way to obtain eleven by adding one-third of twelve to four-fifths of seven—
TW(EL)VE + S(EVEN) = ELEVEN
5.
Here is the completed sum:—
| 2 | 1 | 5 | ) | * | 7 | * | 9 | * | ( | 1 | * | * | 2 | 1 | 5 | ) | 3 | 7 | 1 | 9 | 5 | ( | 1 | 7 | 3 | |
| * | * | * | 2 | 1 | 5 | |||||||||||||||||||||
| * | 5 | * | 9 | 1 | 5 | 6 | 9 | |||||||||||||||||||
| * | 5 | * | 5 | 1 | 5 | 0 | 5 | |||||||||||||||||||
| * | 4 | * | 6 | 4 | 5 | |||||||||||||||||||||
| * | * | * | 6 | 4 | 5 | |||||||||||||||||||||
The clue is that no figure but 3, when multiplied into 215, produces 4 in the tens place.