First of all, this is the chemistry section, while your question is a physics question. Anyway I'll tell you how to solve it.
First we need to find the rate that the truck moves in a second.
1km = 1000m
40km = 40000m
40000m/hr
1 hour = 60 minutes
40000m/hr ÷ 60 = 666.66(repeating)/minute
1 minute = 60 seconds
666.66m/min ÷ 60 = 11.11(rep)m/s
Next we simply multiply the speed of the truck by the number of seconds it travels.
11.11 × 5 = 55.55
Make sure to round it unless you indicate the repeating decimal.
The truck moved 55.56m in 5 seconds.
We know that the equation for density is:

where D is the density, m is the mass in grams, and V is the volume.
Given two of the variables, we can then solve for density:

So therefore, we now know that the density of carbon dioxide gas is 0.00196g/mL.
There is an unstable ratio of protons and neutrons. Because protons are positively charged but neutrons have no charge, an increase in the number of protons means there needs to be an increase in the number of neutrons to "bind" the nucleus together. This is because like charges repel, so the protons will repel each other, and if there aren't enough neutrons to act as "glue" to hold the nucleus together, the nucleus will break apart.
Answer: The momentum of the child and milk together is 58.125 kg.m/s
Explanation:
Momentum is defined as the product of object's mass and velocity.
Mathematically,

where, p = momentum
m = mass of the object
v = velocity of the object
In the given question, we are given that a child of mass 21.0 kg is carrying a gallon of milk having mass 2.25 kg and running with a velocity of 2.5 m/s. Hence, the momentum by both milk and child will be:
....(1)
Given:

Putting values in equation 1, we get:

Hence, the momentum of the child and milk together is 58.125 kg.m/s