15 arches decreased by 80%

3 arches

Step-by-step explanation:

Given

15 arches decreased by 80%.

First get 80% of 15 arches

That's 80%/100% x 15

0.8 x 15

12 arches

Now subtract 12 arches from 15 arches to get the decrease

Decrease = (15 - 12) arches

= 3 arches

Therefore, 80% decrease of 15 inches is equal to 3 arches

If it decreases by 80٪, then only 20٪ remains.

15*20٪=15*20/100=15*0.2=3

Answer: 3