To find the number of transfer units (NTU) of a counter flow heat exchanger, we need to use the effectiveness-NTU method.

Given data:
Effectiveness (ε) = 0.8
Mass flow rate of hot fluid (mh) = Mass flow rate of cold fluid (mc)

The effectiveness of a heat exchanger is defined as the ratio of the actual heat transfer rate to the maximum possible heat transfer rate. It is given by the equation:

ε = (Qactual / Qmax)

The number of transfer units (NTU) is a dimensionless parameter that represents the heat transfer capacity of a heat exchanger. It is given by the equation:

NTU = (UA / Cmin)

where U is the overall heat transfer coefficient, A is the heat transfer surface area, and Cmin is the minimum heat capacity rate of the fluids.

In a counter flow heat exchanger, the hot and cold fluids flow in opposite directions, resulting in a temperature difference between the two fluids at any point along the heat transfer surface. The effectiveness can be calculated using the formula:

ε = (1 - exp(-NTU*(1 - Cr))) / (1 - Cr*exp(-NTU*(1 - Cr)))

where Cr is the capacity ratio, defined as the ratio of the heat capacity rate of the cold fluid to the heat capacity rate of the hot fluid.

In this case, since the hot and cold fluids are both air with equal mass flow rates, their heat capacity rates are also equal, and thus Cr = 1.

Now, we can rearrange the effectiveness equation and solve for NTU:

ε = (1 - exp(-NTU*(1 - Cr))) / (1 - Cr*exp(-NTU*(1 - Cr)))

0.8 = (1 - exp(-NTU*(1 - 1))) / (1 - 1*exp(-NTU*(1 - 1)))
0.8 = (1 - exp(0)) / (1 - 1)
0.8 = (1 - 1) / (1 - 1)
0.8 = 0 / 0

Since we cannot divide by zero, the equation is undefined for ε = 0.8 and Cr = 1. Therefore, there must be an error in the given data or calculations. Please recheck the values provided and try again.

If the correct values are provided, then the calculation may be impossible using the given data.