( a + b + c ) : 3 = 10, 5
( a + b ) : 2 = 12, 25
b = 2 + a
a + b + c = 10, 5 × 3
a + b + c = 31, 5
a + b = 12, 25 × 2
a + b = 24, 5
c = 31, 5 - 24, 5
c = 7
a + 2 + a + 7 = 31, 5
2 × a + 2 + 7 = 31, 5
2 × a + 2 = 31, 5 - 7
2 × a + 2 = 24, 5
2 × a = 24, 5 - 2
2 × a = 22, 5
a = 22, 5 : 2
a = 11, 25
b = 2 + 11, 25
b = 13, 25