Answer: To express the reduction as a rate of kobo in naira, we need to calculate the difference between the original price and the new price, and then convert that difference to kobo.
The original price was 5 naira, which is the same as 500 kobo (since 1 naira = 100 kobo). The new price is 4 naira and 20 kobo, which is the same as 420 kobo.
The reduction in price is the difference between the original price and the new price:
500 kobo - 420 kobo = 80 kobo
So the reduction in price is 80 kobo.
To express the reduction as a rate of kobo in naira, we need to divide the reduction (in kobo) by the original price (in naira) and then multiply by 100 to get a percentage:
(reduction in kobo / original price in naira) x 100
Substituting the values we have:
(80 kobo / 500 kobo) x 100 = 16%
Therefore, the reduction in price is 16% expressed as a rate of kobo in naira.