Linear Equations Questions

What is the percentage of a cloth that should be excluded from $40\text{metre}$ cloth so that the remaining portion is $7\text{metre}$?

In a group of cows and hens, the number of legs are $14$ more than twice the number of heads. How many cows are there?

The age of father is twice that of elder son. Ten years hence, the age of the father will be three times that of the younger son. If the difference of ages of the two sons is $15$ years, find the age of the father.