Write an algebraic expression that has three terms. two of the terms should be variable terms, and one of the terms should be the constant 6

The height of a tree is 10
10
meters long and it grows 1
1
cm in a year.
Then its height after one year = 10
10
meters 1
1
cm
Its height after 3
3
years = 10
10
meters 3
3
cm
Its height after 6
6
years = 10
10
meters 6
6
cm
Its height after x
x
years = 10
10
meters x
x
cm

Where x
x
represents an unknown number. From the last line, we can find height of the tree after a certain number of years by taking x
x
equal to that number. For example, we simply let x=15
x
=
15
, 25
25
and 55
55
. Thus, the value of x
x
depends on our choice. We can give x
x
any value or number we want. In other words, the value of x
x
is not fixed, it varies from one situation to the other. Therefore, we call x
x
a variable whereas 10
10
is a fixed number whole value does not change. So 10
10
therefore is called a constant.

Read more: http://www.emathzone.com/tutorials/basic-algebra/algebraic-expression.html#ixzz4fDkFcbpZ

20 percentage, you can also do it on a calculator.