Why do this problem?
can be approached in several different ways (by randomly trying numbers, working in a systematic way to try numbers, making a table ...) and could be a nice way of introducing algebra.
There are two constraints in this question - the relationship between the number of items bought and the total price.
As an introduction you might wish to start with a similar problem and focus on just one constraint. Choose two similar objects - perhaps a dice and a ruler, and allocate them different prices, say 20p and 30p. Ask the children how many objects you could buy if you had, say, £2.00 to spend.
After they have worked for a while, bring the class together and record their results on the board. (You might do this in a table, or randomly record them and ask the children whether there mght be a better way.)
Ask some questions - what's the greatest number of items I could have bought? The smallest?
Then offer the children the task as written and some time to explore.
Bring them back together again and share their results. If appropriate, you could compare approaches, or together take a look at the submitted solutions and discuss the merits of each.
What do we know?
What could we try first?
How many pencils might she have bought? How many pens would that mean she'd bought?
What could we try next?
Learners could try Buckets of Thinking
which involves similar mathematics.
Using real pencils and pens may help children keep track of what they have tried.