Explanation:

Volume of right circular cylinder = $$\pi r^2h$$
Radius of a right circular cylinder is decreased by 10%, and the height is increased by 20% so,
r1 = r $$\times 90/100$$ = 0.9r
h1 = h $$\times 120/100$$ = 1.2h
Volume of new right circular cylinder =$$\pi r1^2h1$$ = $$\pi (0.9r)^2(1.2h)$$ = 0.972($$\pi r^2h$$)
Decrements in volume = $$\pi r^2h$$ - 0.972($$\pi r^2h$$) = 0.028($$\pi r^2h$$)
Percentage Decrements in volume = $$\frac{0.028(\pi r^2h)}{(\pi r^2h)} \times 100$$ = 2.8%