An architect must choose between installing a low-flow toilet (13 liters/flush) and an ultra-low flow toilet (6 liters/flush). The low-flow toilet will cost $90 while the ultra-low flow toilet costs $165. The cost of water is $1.75/350 liters and both toilets have a life of 10 years. Compute the present worth of both devices if interest is 10% and the average number of flushes is 12 flushes/day.
A) Compute the present worth of the low-flow toilet choice. Do not include a dollar sign in your answer.
B) Compute the present worth of the ultra low-flow toilet choice. Do not include a dollar sign in your answer.
1. Low-flow toilet choice:
No. of litres of water per day = 12 flushes x 13 litres = 156 litres
No. of litres of water per year = 365 days x 156 litres = 56,940 litres
Cost of water = 56,940 litres x $1.75/350 litres
= $284.70
Initial Cost = $90
Annual cost of water = $284.70
Annuity factor of 10% for 10 years = 6.1446
Present Worth = $90 + $284.70 x 6.1446 = $90 + $1,750 = $1,840
2. Ultra Low-flow toilet choice:
No. of litres of water per day = 12 flushes x 6 litres = 72 litres
No. of litres of water per year = 365 days x 72 litres = 26,280 litres
Cost of water = 26,280 litres x $1.75/350 litres
= $131.40
Initial Cost = $165
Annual cost of water = $131.40
Annuity factor of 10% for 10 years = 6.1446
Present Worth = $165 + $131.40 x 6.1446 = $165 + $807 = $972
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