Question

Julio's father is 4 times as old as Julio. The sum of their ages is no less than 55.

Answer 1

Julio is x years old and his father is y years old:
y = 4 x
x + y ≥ 55
x + 4 x ≥ 55
An inequality is:
5 x ≥ 55
x ≥ 55 : 5
x ≥ 11
The youngest age Julio can be is 11.

Answer 2

Call F the age of the father and J the age of Julio.

The F and J are related in this way: F = 4J

Now you have a restriction in the form of inequality: The sum of both ages has to be greater or equal than 55.

Algebraically that is: F + J ≥ 55

You can substitute F with 4J to find the solution for J:

4J + J ≥ 55

5J ≥ 55

Now divide both sides by 5

5J/5 ≥ 55/5

J ≥ 11

That imposes a lower boundary for the value of J of 11, meaning that the youngest age of Julio can be 11.