Sales Planning and Operational Variance

Dear Sir,

I came across one question in BPP learning material on Planning and operational variance analysis (Activity 3) . The question says as below

Budget production and sales – 5000 units
Actual production and sales – 5200 and 5100 respectively
You discovered that industry sales of Chils were 10% lower than forecast

Required to analyze whether there was a favourable or Adverse Sales Volume planning and operational variance.

In answer, they have calculated revised sales volume as 4500 units. My question is

How have they derived this 4500 quantity as in above it says that sales were 10% lower than forecast!!

Please help me understand the above

Regards

Aasif