let

x = the tens digit

y = the ones digit

The sum of the digits of a two-digit number is 11:

x + y = 11

The tens digit is one less than three times the ones digit:

x = 3y - 1

by solving the system of equations:

x + y = 11

x = 3y - 1

we find:

x = 8

y = 3

click here to see the step by step solution of the system of equations:

the original number is 83.