23) the ring has a radius of 6mm and is 2 mm wide, so it has an outer ring at 7mm and an inner one at 5mm to get the area subtract the inner ring from the outer, because of the hole: outer=7²*π=49π inner=5²*π=25π ring area=outer-inner=49π-25π=24π= option C
24)circumference of a circle is c=2rπ
for the larger one: 2*20*π=40π smaller one:2*15*π=30π
distance the larger wheel moves in 30 revolutions: circumference*30=40π*30=120 π
revolutions of the smaller wheel by dividing the distance with the circumference: 120π/30π=40 revolutions=option H
