Answer:
7
Step-by-step explanation:
Let the middle number be x.
Since the middle number is 1/3 the last number, then the last number has to be three times the middle number,
Therefore the last number is 3x
The first is 3 times the last number, meaning the first number has to be 3*3x=9x
All the three number add up to 91,
thus 9x+x+3x=91
13x=91
x=91/13=7