Let L, M, C represent the numbers of messages each sent.
L+M+C=139 total messages sent.
C=4L, L=M-7, so M=L+7. Now substitute for C and M:
L+L+7+4L=139,
6L+7=139,
6L=132, L=132/6=22, therefore C=4L=88, M=L+7=29.
Lucy sent 22 messages, Milan 29 messages, Carlos 88 messages.