Solve the mathematical puzzle.

Question

Determine the digits of R from these clues.

The first digit is the answer when the third digit is divided by 2.

The second and third digits add to 7.

The second digit minus the first digit is 1.

R is a three-digit number: ____

I have never understood how to do this.

Answer 1

Call the digits a,b,c left-to-right

a = c/2
b+c = 7
b-a = 1

we see that c=2a, so

b+2a=7
b-a=1
subtract to get
3a=6
a=2
Thus c=4 and b=3
R=234

Or, just working with the specs,
c must be even, since it can be divided by 2. So, R is one of the forms
1x2
2x4
3x6
4x8
Since the last two add to 7, R must be
152
234
316

Since 2nd-1st=1,
R=234