geometric progression word problem,

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A liquid is kept in a barrel. At the start of a year the barrel is filled with 160 litres of the liquid. Due to evaporation, at the end of every year the amount of liquid in the barrel is reduced by 15% of its volume at the start of the year.

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(a) Calculate the amount of liquid in the barrel at the end of the first year.

Liquid at the start of the year = 160

end of 1st year = 160x0.85 = 136ml

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(b) Show that the amount of liquid in the barrel at the end of ten years is approximately 31.5 litres.

Liquid at end of year 10=160×(0.85)^10=31.499 … =31.5 litres

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At the start of each year a new barrel is filled with 160 litres of liquid so that, at the end of 20 years, there are 20 barrels containing liquid.

(c) Calculate the total amount of liquid, to the nearest litre, in the barrels at the end of 20 years.

need help with part 'c'

thanks.