Here is a probability scale: 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

There are 3 white counters and 1 black counter in a bag.
I take one of the counters at random.
Put a cross on the probability scale to show the probability
that I have chosen the black counter.

The probability of choosing a black counter is 0.25.

Also the picture is attached for the mark on scale.

Step-by-step explanation:

Probability is the likeliness of occurrence of an event.

Given

There are three white and 1 black counter in the bag which means our sample space contains 3+1 = 4 elements

n(S) = 4

Let B be the event that the drawn out counter is black. As there is only one black counter

n(B) = 1

The probability of choosing a black counter is:

Hence,

The probability of choosing a black counter is 0.25.

Also the picture is attached for the mark on scale.

Dividing a number by 10 is probably the easiest thing to do, you just need to follow these rule :

1. If the number that has zero divided by 10, you just need to take out 1 zero
Example : 200 : 10  = 20

2. If the number that doesn't has zero divided by 10, you just need to put a comma in front of the last number
For example : 1998 : 1  = 199.8

hope this helps