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Successive discounts of p%p\% and q%q\% on an invoice is the same as a single discount of

Solution

✅ Correct Option: 1

Take the original price as 100, since we want the answer as a percentage.

After the first discount of p%p\%, the price left is

100−p100-p

The second discount of q%q\% applies to this reduced price:

(100−p)−q100(100−p)(100-p)-\dfrac{q}{100}(100-p)

=(100−p)(1−q100)=(100-p)\left(1-\dfrac{q}{100}\right)

=100−p−q+pq100=100-p-q+\dfrac{pq}{100}


Total discount=starting price−final price\text{Total discount}=\text{starting price}-\text{final price}

=100−(100−p−q+pq100)=100-\left(100-p-q+\dfrac{pq}{100}\right)

=p+q−pq100=p+q-\dfrac{pq}{100}

Since we started from 100, this number is already the percentage:

[p+q−pq100]%\left[p+q-\dfrac{pq}{100}\right]\%

The minus sign makes sense, because the second discount is taken on a smaller price, so you save a little less than p+qp+q.

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