Let M, A, J and S be the corresponding probabilities for each of the participant.
Then from the condition, you have this system of 4 equations
M + A + J + S = 1 (1)
M = 3A (2)
J = 2S (3)
M + J = 0.7 (4)
The translation is just completed.
Note that (4) is true since the events to win for M and for J are DISJOINT.
The rest is just technique, and I do not want to deprive you a joy to complete the solution on your own.