X.

is $35$ years old, and his son is one-fourth of that age. In how many years will the son be half as old as his

5. A is $60$ years old, and B's age is two-thirds of A's. How many years ago was B's age one-fifth of A's?

6. A son is one-third as old as his father. Four years ago he was only one-fourth as old as his father. What is the age of each?

7. A is $50$ years old, and B is half as old as A. In how many years will B be two-thirds as old as A?

8. B is one-half as old as A. Ten years ago he was one-fourth as old as A. What are their present ages?

9. The sum of the ages of a father and his son is $80$ years. The son's age increased by $5$ years is one-fourth of the father's age. Find their ages.

Exercise 58.

Ex. A can do a piece of work in $2$ days, and B can do it in $3$ days. How long will it take both together to do the work?

lint Let x = the number of days it will take both together. lint Then 1x = the part both together can do in one day, 12 = the part A can do in one day, 13 = the part B can do in one day, lintand 12 + 13 = the part both together can do in one day.