So there are 10 combinations of A and B * * *.
(3, 12), (4, 1 1), (5, 10), (6,9), (7,8), (8,7), (9,6), ( 10,5), ( 1 1,4), ( 12,3)
For x 1, x2
When a=3, (x 1, x2)=( 1, 2) or (2, 1), there are two combinations.
When a=4, (x 1, x2)=( 1, 3) or (2,2) or (3, 1), there are three combinations.
When a=5, (x 1, x2)=( 1, 4) or (2,3) or (3,2) or (4, 1), there are four combinations.
When a=6, (x 1, x2) has five combinations.
When a=7, (x 1, x2) has six combinations.
When a=8, pay attention to x 1, x2.
When a=9, note x 1, x2.
When a= 10, (x 1, x2) has three combinations.
When a= 1 1, (x 1, x2) has two combinations.
When a= 12, (x 1, x2) = (6,6), there are only 1 combinations.
When b=n, the corresponding number of combinations should be the same as when a = n.
So the total number of combinations should be
=2× 1
+3×2
+4×3
+5×4
+6×5
+5×6
+4×5
+3×4
+2×3
+ 1×2
= 140 (type)