Megan walked a total of 4 kilometers by making 2 trips to school. How many trips will Megan have to make in all to walk a total of 18 kilometers? Assume the relationship is directly proportional.

Ron jarred 12 liters of jam after 4 days. How many days does Ron need to spend making jam if he wants to jar 18 liters of jam in all? Assume the relationship is directly proportional.

Spencer took a total of 8 quizzes over the course of 2 weeks. How many weeks of school will Spencer have to attend this quarter before he will have taken a total of 12 quizzes? Assume the relationship is directly proportional.

Isabelle bought 6 souvenirs during 2 days of vacation. How many days will Isabelle have to spend on vacation before she will have bought a total of 9 souvenirs? Assume the relationship is directly proportional.

Submarines control how much they float or sink in the ocean by changing the volume of air and water contained in large ballast tanks. when the tanks are completely full of water, the submarine increases its overall mass and sinks down to the bottom. when the tanks are completely full of air, the submarine reduces its overall mass and floats to the surface. depending on the density of the seawater surrounding the submarine, it will pump seawater in or out of the tanks in order to achieve the same overall density as the sea water and float neutrally in the water. the volume of the submarine never changes. when the tanks are completely full of water, a submarine with a volume of 7.8\times10^3\text{ m}^37.8×10 3 m 3 7, point, 8, times, 10, start superscript, 3, end superscript, space, m, start superscript, 3, end superscript has a total mass of 8\times10^6\text{ kg}8×10 6 kg8, times, 10, start superscript, 6, end superscript, space, k, g. the density of the seawater is 10^3\text{ kg/m}^310 3 kg/m 3 10, start superscript, 3, end superscript, space, k, g, slash, m, start superscript, 3, end superscript. to make that submarine float neutrally, and neither float nor sink in the ocean, what volume of water does that submarine need to subtract from its tanks?