The speed limit on many interstate high ways in the US.
65 miles per hour. How many
kilometers per hour is that?(1 mile= 1.61km)

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s

i would say

conservation of mass

mass before something happens = mass afterwards