A=?;B=?

3a=78-2b
a=(78-2b)/3
[(78-2b)/3] x2 +3b=72
(78-2b)x2+9b=216
156+5b=216
5b=60
b=12
a=18

A+A+A+B+B=78
3A + 2B = 78
3A = 78 - 2B
A = (78-2B)/3

A+A+B+B+B=72
2A + 3B = 72
2[(78-2B)/3] + 3B = 72-------[A = (78-2B)/3]
(156-4B)/3 + 3B = 72
156-4B + 9B = 72 x 3--------(兩邊各乘3)
156 + 5B = 216
5B = 216 - 156
B = 60/5
B = 12

A = (78-2B)/3
A = [78-2(12)]/3
A = (78-24)/3
A = 54/3
A = 18

3A + 2B = 78
2A + 3B = 72
-------------------
A - B = 6

A = B - 6

3A + 2B - 12 = 78
5A = 90
A = 18
B = 24

A+A+A+B+B=78
A+A+B+B+B=72

3A+2B=78......(1)
2A+3B=72......(2)
(1)*2-(2)*3: 4B-9B=6
5B=6
B=6/5
Sub B=6/5 into (1)
3A+2(6/5)=78
3A+12/5=78
3A=378/5
A=126/5
Therefore A=126/5=25.2, B=6/5=1.2