a and b represent two rational numbers, and the new operation "*" is stipulated as: a*b=ma+2b (where m is a rational number), if 1*2=5, then what is the value of m.
a、b表示两个有理数，规定新运算"*"为：a*b=ma+2b(其中m为有理数)，如果1*2=5，那么m的值为多少．
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