In maths we sometimes use a letter to represent an unknown amount. This is algebra.
We can make algebraic expressions by adding, subtracting, multiplying or dividing this letter.
We often use ? or n as the letter.
Paul has n sweets (this means we don't know how many sweets Paul has - we'll just call it n).
Jim has 2 more sweets than Paul, and Molly has 5 less than Paul.
An expression for the number of sweets that Jim has is n+2. An expression for Molly is n-5.
Jane has ? cousins. Mary has twice as many as Jane, but Keith only has half as many as Jane.
An expression for the number of cousins that Mary has is 2? (this means 2 times ? , but we don't write the times sign because it gets confused with ? Keith has ?|2 (this means ? divided by 2).