Question

Becky has some marbles

Answer 1

Becky - x marbles;
Chris - y marbles;
Dan - z marbles
y = 2 x
z = y + 7
x + y + z = 57
--------------------
z = 2 x + 7
x + 2 x + 2 x + 7 = 57
5 x + 7 = 57
5 x = 57 - 7
5 x = 50
x = 50 : 5
x = 10,  y = 20,  z = 27
Answer: Dan is not correct. 

Answer 2

Chris - y marbles

Becky - x marbles

Dan - z marbles

y = 2 x

z = y + 7

x + y + z = 57

z = 2 x + 7

x + 2 x + 2 x + 7 = 57

5 x + 7 = 57

5 x = 57 - 7

5 x = 50

x = 50 : 5

x = 10,  y = 20,  z = 27

This means that Dan is not correct.