Letter number letter
Number number letters number
Number letters numbers letter letters

Letter - number + letter
Number - number + letters + number
Number - letters - numbers - letter - letters + numbers

However, this skill is very important to handle questions with UNITS and PARTS. 
The concept of units and parts is VERY IMPORTANT for problem sums.

The good news is: NOT TO WORRY
If your child makes mistakes on this, he just needs more practice until it CLICKS in his brain.

Answer for this question will be posted tonight. :)

To try similar questions, click url for practice: 
http://bit.ly/ischool-P6AG17080406

Topic: Algebra
Level: Primary 6

Concepts:
Algebra in one variable
Algebra simplifying

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iSchool Math: Best Singapore Math Portal for Problem Sums for higher-order thinking

The solution: