(a) The GST amount included in the invoice is **$100**.

Working: GST-inclusive price = $1,100. GST rate = 10%.
GST amount = $1,100 × (10/110) = $1,100 × 1/11 = **$100**

(b) The GST-exclusive price is **$1,000**.

Working: GST-exclusive price = GST-inclusive price − GST amount = $1,100 − $100 = **$1,000**

Alternatively: GST-exclusive price = $1,100 × (100/110) = $1,100 × 10/11 = **$1,000**