Question 386758
First, we'll assign variables to both ages.
let M=Mona's age
let B=Bailey's age

Then, we'll come up with two equations to solve our two variables.
Mona's age is 4 times Bailey's age. This can be shown algebraically as below:
M=4B

We also know that Mona's age plus Bailey's age is 15. This, in an algebraic statement, can be written as below:
M+B=15
We can substitute the value of M in terms of B from the first equation into the second equation:
4B+B=15
5B=15
Then, we can divide both sides of the w equation by 5:
{{{5B/5=15/5}}}
B=3

Therefore, Bailey is 3 years old. We also want to know Mona's age, so we can simply plug in Bailey's age to either equation and find it. 
M+B=15
M+3=15
M=12
Therefore, Mona is 12 and Bailey is 3.