What is the unit rate of 15 laps in 3 days

Which equation represents the partial sum of the geometric series?

Consider a cube that has sides of length l. now consider putting the largest sphere you can inside this cube without any point on the sphere lying outside of the cube. the volume ratio -volume of the sphere/ volume of the cube is 1. 5.2 × 10−1 2. 3.8 × 10−1 3. 1.9 4. 2.5 × 10−1 5. 3.8

Given the two similar triangles, how do i find the missing length? if a=4, b=5, and b'=7.5 find a'