Answer: $90
Explanation: This problem can be solved by using following equation :-
Let John's hourly wage rate be J, Mary's hourly wage rate be M and Dennis hourly wage rate be D, therefore :-
Mary's rate will be :-
M = 2J............equation 1
AND,
J + D = $60 ..... equation 2
Similarly,
D = 1/3J
Now,putting the value of D in equation 2 we get,
J + 1/3J = $60
J = $45
Putting the values of J in equation equation 1 we get,
M = 2 * $45
= $90
So, Mary's hourly wage rate is $90