15. Determine the mass to cause the seesaw to be balanced. Assume the seesaw is uniform in mass and 8 meters long. The mass on left is 15 kg. The mass of the seesaw is 5 kg. Assume boxes have negligible width. ? X 2.5 m

I went through the math but the awnser is still marked as incorrect

NO NEED TO SHOW ANY WORK JUST FINAL ANSWER AS QUICKLY AS POSSIBLE! Incredibly, the ozone layer in the upper atmosphere that protects us from the ultraviolet rays of the Sun varies somewhat but, on the average, would be less than one centimeter thick if it were brought down to sea level. Given that the density of ozone at sea level would be about 2.0 grams per litre, what is the total amount of ozone (in Tonnes) in the upper atmosphere when its sea-level thickness is 0.22cm? Take the radius of the Earth to be 6400 km. (The volume of a thin skin around an object is the surface area of the object times the skin thickness.) Give your answer in gigatonnes, and do not specify units in the answer box.

A hot tub with a surface area of 28 ft2 is filled with water to a depth of 29 in . Hint: volume is calculated as area × height (A × h). A) What is the volume of water in the tub, in liters? Express your answer to two significant figures and include the appropriate units. B) How many kilojoules are needed to heat the water from 59∘F to 103 ∘F? Express your answer to two significant figures and include the appropriate units. C) If the hot-tub heater provides 5900 kJ/min, how long, in hours, will it take to heat the water in the hot tub from 59∘F to 103∘F? Express your answer to two significant figures and include the appropriate units.