Question 944760
M is Mary's age. J is Joseph's age
J=2M; J+M=51 Substitute first equation for J in second equation
2M+M=51
3M=51 Divide by 3
M=17 ANSWER 1: Mary is 17
J=2M
J=2(17)
J=34  ANSWER 2: Joseph is 34
CHECK 
M+J=51 
17+34=51
51=51