Answer:
Take £ to be sigma 
mean=£fx/£f
This means the sum of the digits divided by the number of digits 
So from the above question 10+12+20+23+25=90 but 90 - 10 = 80
And 80 divided by 4 is 20 
therefore the mean is 20.
Step-by-step explanation:
 First of all, since the sum of the above digits is 90, we would need to exclude one of the above digits to make the number, a number divisible by 4( since we have excluded one of the 5 digits to make it 4 ) equal to 20. 
So systematically, the equation would be defined as 
x/4 = 20/1 
hence we cross multiply and x becomes 
x = 20 × 4 
therefore x= 80 
Hence, since x = 80 we divide this by our number of digits above hence giving you 
80 ÷ 4 = 20 
So from the above equation, we would need to exclude 10 to give us our mean of 20.