Two people are selected at random from a group of 6 pilots and 4 engineers. what is the probability that both of them are engineers?

Probability is the ratio of: the number of observations of an event n(A) compared to the total number of observation n(S). Generally it could be written as
P(A) = 

Use combination to solve this problem
The event being observed n(A) in this problem is 2 engineers being selected from 4 engineers. The observation n(S) is about 2 people being selected from 10 people (engineers + pilots).

n(A) could be determined using combination of 2 engineers from 4 engineers
n(A) = 
n(A) = 
n(A) = 
n(A) = 
n(A) = 
n(A) = 6

n(S) could be determined using combination of 2 people from 10 people
n(S) = 
n(S) = 
n(S) = 
n(S) = 
n(S) = 
n(S) =
n(S) = 45

The probability
P(A) =
P(A) =
P(A) =

The probability is 2/15

Aand b are correctly balanced reactions. they have the same number of atoms in both sides of the reaction