I&#x27;m not quite sure where you&#x27;re going with k vs k+1.<p><pre><code>  we know that s + d = 209 (1)
  and we know that 6.5s + 2d = 836.5 (2)
  we can rewrite (1) as d = 209 - s (3)
  substituting (3) into (2) we  get 6.5s + 2(209 - s) = 836.5 (4)
  solving (4) for s yields s = 93 (5)
</code></pre>
alternatively, you can solve the system of linear equations<p><pre><code>  [   1 1 ] [ s ] = [   209 ]
  [ 6.5 2 ] [ d ] = [ 836.5 ] (1)
  
  [ s ] = [ -4&#x2F;9  2&#x2F;9 ] [   209 ]
  [ d ] = [ 13&#x2F;9 -2&#x2F;9 ] [ 836.5 ] (2)
  
  [ s ] = [ 93  ]
  [ d ] = [ 116 ] (3)</code></pre>