Answer:
Step 4
Step-by-step explanation:
Given: Steps used by Shelly to evaluate the expression
To find: step that includes Shelly's mistake
Solution:
According to PEMDAS,
P denotes Parentheses
E denotes Exponents
M denotes Multiplication
D denotes Division
A denotes Addition
S denotes subtraction
Step 3 is
and step 4 is 
In step 4, first subtraction operation has been applied before the division operation.
There is a mistake in step 4 as according to PEMDAS, first -20 should be divided by 5 then 1 should be subtracted from the resultant number.
Answer:
Third option : 9^10 / 9^2
Step-by-step explanation:
9^2 . 9^6
Formula / Identity : -
a^m . a^n = a^( m + n )
Here,
a = 9
m = 2
n = 6
9^2 . 9^6
= 9^( 2 + 6 )
=9^8
9^10 / 9^2
Formula / Identity : -
a^m / a^n = a^( m - n )
Here,
a = 9
m = 10
n = 2
9^10 / 9^2
= 9^( 10 - 2 )
= 9^8
Therefore,
9^10 / 9^2 is equivalent to 9^2 . 9^6.
15a. pentagon
15b. parallelogram
15c. square
Hope this helps :D