*Equation 1*
{latex}
$$
\displaylines{
1 - {{NTU} \over {NTU_{\max } }} = {t \over {K + t}} \cr
{{NTU} \over {NTU_{\max } }} = {K \over {K + t}} \cr}
$$
{latex}

*Equation 2*
{latex}
$$
v = {L \over t}
$$
{latex}

substitute the equation (2) in equation (1):
*Equation 3*
{latex}
$$
\displaylines{
  1 - {{NTU} \over {NTU_{\max } }} = {L \over {Kv + L}} \cr 
  {{NTU} \over {NTU_{\max } }} = {{Kv} \over {Kv + L}} \cr} 
$$
{latex}

Double reciprocal equation
*Equation 4*
{latex}
$$
{1 \mathord{\left/
 {\vphantom {1 {\left( {1 - {{NTU} \over {NTU_{\max } }}} \right)}}} \right.
 \kern-\nulldelimiterspace} {\left( {1 - {{NTU} \over {NTU_{\max } }}} \right)}} = {{Kv} \over L} + 1
$$
{latex}