Probability

If a number is randomly selected from 1-80,find the probability that the number
selected gives a remainder 1 when it is divided by 4.
If possible, list all the possible outcomes.
Sol
x/4=___________......1
x=4a+1
1<=x<=80
1<=4a+1<=80
0<=4a<=79
0<=a<=19.75
0<=a<=19
19－0+1=20
p=20/81
x=1 or 5 or 9 or 13 or 17 or 21 or 25 or 29 or33 or 37 or 41 or 45
or 49 or 53 or 57 or 61 or 65 or 69 or 73 or 77

There are only 80 numbers from 1 - 80.
Hence, the required probability = 20/80 = 1/4

"1" is a favourable outcome because :
1 ÷ 4 = 0 ...... remainder 1