Question 296012
<pre><font size = 4 color = "indigo"><b>
The number "1A" is {{{10+A}}}
The number "2B" is {{{20+B}}}

So since 

"1A" + A = "2B",

{{{(10+A)+A=20+B}}}
{{{10+A+A=20+B}}}
{{{10+2A=20+B}}}
{{{2A=10+B}}}
{{{B=2A-10}}}
{{{B=2(A-5)}}}
{{{0<=B<=9}}}
{{{0<=2(A-5)<=9}}}
{{{0<=2A-10<=9}}}
Add 10 to all three sides:
{{{10<2A<=19}}}
{{{5<=A<=9.5}}}
{{{5<=A<=9}}}

So the different digits from 0 through 9 which A could represent are 
the five numbers 5,6,7,8,9 

So the answer if five.

If A=5, then B=2A-10 = 2(5)-10 = 10-10 = 0

15
+5
--
20

If A=6, then B=2A-10 = 2(6)-10 = 12-10 = 2

16
+6
--
22

If A=7, then B=2A-10 = 2(7)-10 = 14-10 = 4

17
+7
--
24

If A=8, then B=2A-10 = 2(8)-10 = 16-10 = 6

18
+8
--
26

If A=9, then B=2A-10 = 2(9)-10 = 18-10 = 8

19
+9
--
28

Edwin</pre>