Answer:
a = 1/42; b = 1/7; c = 1/3; d = 1/2
Step-by-step explanation:
a = bcd
a + b = cd
a + b + c = d
a + b + c + d = 1
d + d = 1
2d = 1
d = 1/2
2a = bc
2a + 2b = c
2a + 2b + 2c = 1
bc + 2b = c
bc + 2b - c = 0
bc + 2b + 2c = 1
-3c = -1
c = 1/3
a + b = cd
a + b = 1/6
a = bcd
a = b(1/3)(1/2)
a = 1/6 b
1/6 b + b = 1/6
7/6 b = 1/6
b = 6/42
b = 1/7
a = 1/6 b
a = 1/6 * 1/7
a = 1/42
