Ian earns £420 a week after a 5% rise. What was his pay before?

Ian had a 5% rise, so £420 = 105% of his original pay 

To find 1% of his original earnings, divide £420 by 105.

Multiply this by 100 to find his old pay: 
1% of his original earnings = 420/105 = £4 

Original earnings = £4 × 100 = £400