Understanding the Problem

We need to find the temperature on Friday. We are given the average temperature for three days (Friday, Saturday, Sunday) and the average temperature for another three days (Saturday, Sunday, Monday). We also know the exact temperature on Monday.

Calculating Friday's Temperature

1. Let the temperatures be represented by variables: $F$ (Friday), $Sa$ (Saturday), $Su$ (Sunday), and $M$ (Monday).

2. Write the equations based on the given averages:

   • Average (Fri, Sat, Sun): $\frac{F + Sa + Su}{3} = 26^\circ$
   • Average (Sat, Sun, Mon): $\frac{Sa + Su + M}{3} = 24^\circ$

3. Calculate the total sum of temperatures for each period:

   • Sum (Fri, Sat, Sun): $F + Sa + Su = 26 \times 3 = 78^\circ$
   • Sum (Sat, Sun, Mon): $Sa + Su + M = 24 \times 3 = 72^\circ$

4. Use the given temperature for Monday ($M = 25^\circ$):

   • Substitute $M$ into the second sum: $Sa + Su + 25^\circ = 72^\circ$
   • Find the sum of Saturday and Sunday temperatures: $Sa + Su = 72^\circ - 25^\circ = 47^\circ$

5. Substitute the sum of Saturday and Sunday temperatures ($Sa + Su = 47^\circ$) into the first sum equation:

   • $F + 47^\circ = 78^\circ$
   • Solve for Friday's temperature: $F = 78^\circ - 47^\circ = 31^\circ$

Therefore, the temperature on Friday was $31^\circ$.