P5/P6 Math: Value and Ratio and Differences

This is a common P5 SA2 and P6 Math question that is usually worth 4 marks or more.
Such questions will appear in Paper 2.
Calculator usage is allowed.
(An easier version of this question can be found here)

Rick spent a total of $31.20 buying some erasers and pens.
The ratio of pens to erasers was 9:2.
The total value of all the pens is $26.40 more than the total value of all the erasers.
Each pen costs $0.50 more than an eraser.

a) How much is an eraser?
b) How many pens did Rick buy?


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