2. As a wave moved from one medium to another, there was a change in the
wave's speed. If the frequency of the
wave does not change, what property of
the wave must change, causing the
change in speed?

that is a hard problem I had the same one.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.

answer: 2×10^-2kg

explanation: density= mass per volume

vol=mass/density

converting to s.i units we have;

3÷1000kg = 0.003

1.5÷100m = 0.15

0.003/0.15 = 0.02kg