Answer:
Formula for mean in grouped data
= Zfx/ Zf
f = sum of the number of mice
= 35
Frequency = 39 + x
Mean = 7
Fx = 20 + 78 + 112 + 8x + 54
= 264 + 8x
7 = 264 + 8x/ 39 + x
7(39+x) = 264 + 8x
After solving you will get
x = 9
Hope this helps.
