A shopkeeper gives a discount for purchases in bulk. If one earaser costs 90cts and a bulk purchase of 10 earasers cost $8,

(a) and (b) are correct.

For (c), if the "normal" price is that of 10 individual erasers (namely, $9) then the discount ($1) as a percentage  of the normal price is given by (1/9)*100, or ≈ 11%

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Thanks Alan. And you too Anonymous.

Yes, you are correct. Because,

Normal Price = $9

Bulk Price = $8

Discount% = ($9 - $8)/10 (Erasers) *100

                    = 1/10 *100

                    = 10%.

The discount is asked for as a percentage of the normal price, not of the number of erasers!

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