Question 1019851
Set M = number of Martians
Set E = number of Earthlings
2*M + E = 29, that is two times number Martians plus the number of Earthlings
5*M + 2E = 67, 5 times the number of Martians plus 2 times Earthlings = no. of legs
2M + E = 29
5M + 2E = 67
Adjust 2M + E = 29 by adding -2M to each side
E = 29 - 2M
Substitute (29 - 2M) for E in 5M + 2E = 67
5M + 2(29 - 2M) = 67
5M + 58 -4M = 67
M + 58 = 67
add -58 to each side
M = 9
Since E = 29 - 2M 
E = 29 - 2(9)
E = 29 - 18
E = 11
Checking our original equations,
2*M + E = 29, 2*9 + 11 = 29 , 18 + 11 = 29, so this checks.
5*M + 2E = 67, 5*9 +2*11 = 67 , 45 + 22 = 67, so this checks.
We have 9 Martians and 11 Earthlings.
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