TL:DR: 14
The easiest way to do this problem is to form a system of equations, and solve for both variables. To do this, we will refer to the ones (or units, depending on regional dialect) digit as y, and the tens digit as x. The first part of the question tells us that the sum of the two digits is five, so that gives us our first equation:
x + y = 5.
The second part of the equation tells us that the units digit is one more than three times the tens digit. So y is equal to 3 times x plus one, or in equation form:
y = 3x + 1
Now we just have to plug one equation into the other. This is made easier by the fact that one of the equations is already solved for a variable. So we just substitute 3x +1 in for y in the first equation:
x + (3x + 1) = 5
4x + 1 = 5
4x = 4
x = 1
So now that we know what x is, we just have to plug it into one of the equations to find y. The top is the easier one to do this with, so that is the one we will plug it into:
x + y = 5
1 + y = 5
y = 5 - 1 = 4
And there we go. The units digit is 4, and the tens digit is 1, so the final number is 14.
(Note: Should you have a graphing calculator, or graph paper, you could also solve both equations for y, and graph them both. The solution to the problem will be the point where the two lines intersect. Although this way is easier, it is still good to know how to do these problems easier.)
