Question 622124
{{{M + 6n = 15.7}}}
{{{M + 11n = 21.7}}} subtract these two
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{{{ -5n = - 6}}}
{{{n = 6/5}}}
Now use that value of n in the first equation to solve for M
{{{M + 6*(6/5) = 15.7}}}
{{{M = 15.7 - 36/5}}}
{{{M = 15.7 - 8.4}}}
{{{M = 7.3}}}

You can check your answer by using M and n in second equation to ee if the result is 21.7