Question 1193092
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1/T - 1/M = 1/R

Add 1/M to both sides, subtract 1/R from both sides:
1/T - 1/R = 1/M

Take reciprocal of both sides:
1/(1/T - 1/R) = M

Re-writing with M on LHS:

{{{ matrix(3,3,"","","", "M", " = ",  1/(1/T - 1/R), "" , "", "") }}}


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Just in case the student is wondering, the other tutor's answer is just a different form for the one I provided:  

  {{{ RT / (R-T) }}}  = {{{  ((RT / RT)) / (((R-T)/(RT))) }}} = {{{ 1 / (1/T-1/R) }}}

Also note the implied restrictions: {{{T<>0}}}, {{{R<>0}}}, {{{ T<>R}}}