Answer: 40 grams
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Step-by-step explanation:
8 jars need 1.8 kg of caster sugar.
Cut each value in half to see that 4 jars need 0.9 kg of caster sugar.
Convert 0.9 kg to grams to get 900 grams (multiply by 1000 to go from kg to grams).
Jayne needs 900 grams of caster sugar. She has 860 grams already, which means she needs an additional 900-860 = 40 grams