Explanation:
GIVEN: Two digit number means we will have to choose from numbers 0-9 for the second digit and 1-9 for the first digit. (since zero can't be in the tens place)
Formula: We will use m x n. That is, m is the number of elements we can use for the first digit and n will be the number of elements we can use for the second digit.
Sample Space: To get the probability, we must find the sample space or all the outcomes.
m - 9 elements (1-9)
m - 9 elements (1-9)n - 10 elements (0-9)
m x n = 9x10 =90
90 is our sample space.
A. An odd number
Conditions: For an odd number, the second number will determine whether or not it is an odd number. For the first digit, we can use 1-9.
Elements:
m - 9 elements (1-9)
m - 9 elements (1-9)n - 5 elements (1,3,5,7,9)
m x n=9x5=45
SOLVE (probability)
Probability= possible outcomes/sample space
P=45/90
P=1/2
B. Larger than 75.
Conditions: The first digit could be equal to 7 or greater. But n will only be greater than 5 if and only if m=7 and n could be any number if m>7.
Elements 1:
m - 1 element (7)
m - 1 element (7)n - 4 elements (6,7,8,9)
m - 1 element (7)n - 4 elements (6,7,8,9)m x n=1x4=4
Elements 2:
m - 2 elements (8,9)
m - 2 elements (8,9)n - 10 elements (0,1,2,3,4,5,6,7,8,9,)
m - 2 elements (8,9)n - 10 elements (0,1,2,3,4,5,6,7,8,9,)mxn=2x10=20
TOTAL: 4+20=24
SOLVE (PROBABILITY)
Probability= possible outcomes/sample space
P=24/90
P=4/15
C. multiple of 5
Condition: the second digit must be either 0 or 5 and the first digit could be any digit.
Elements:
m - 9 elements (1-9)
m - 9 elements (1-9)n - 2 elements (0,5)
m x n =9x2=18
SOLVE (PROBABILITY)
P=18/90
P=1/5
D. An even number smaller than 40
Condition: The first digit could be equal to 4 but the second digit must be even (excluding zero). Another situation is the first digit would be greater than 4 and the second digit would be any even number.
Elements 1:
m - 1 element (4)
n - 4 elements (2,4,6,8)
m x n = 1x4=4
Elements 2:
m - 5 elements (5,6,7,8,9)
n - 5 elements (0,2,4,6,8)
mxn=5x5=25
TOTAL: 25+4=29
SOLVE: Probability
P=29/90