Adrian Mathie, who hasn't given us his age or school unfortunately, has worked out this problem very efficiently:

No of pencils = x
No of pens = 4x

Spent on pencils = 10x
Spent on pens = 4x × 15 = 60x
Total spent = 10x + 60x = 70x

70x = 250 - 40
70x = 210
x = 210 / 70 = 3

Therefore no of pencils = 3
No of pens = 3 × 4 = 12

Pupils at Moorfield Juniors, including Emma, approached it slightly differently:

We worked this out by taking 40p from £2.50 which is 2.10 then using trial and error.
It was easy because there aren't many low numbers in the 4 times table.
The answer is 12 pens which equals £1.80. 12 divided by 4 = 3 so there are 3 pencils which equals 30p.
£1.80+30p=£2.10.