Physics Problems: maths for physics

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I'm learning "Physics Problems: maths for physics" on The Future Market. Lesson 1: "A method for every problem: units, rearranging and significant figures".

What the lesson teaches:
- Five steps for every problem: (1) Draw a sketch and write the given numbers with units. (2) Write the unknown you want. (3) Choose the equation that links them. (4) Rearrange the equation with algebra first, then put in numbers. (5) Check the units and whether the size of the answer makes sense.
- Rearranging: whatever you do to one side of an equation you do to the other. From v = d / t you get d = v x t and t = d / v. For example, 100 m at 8 m/s takes t = 100 / 8 = 12.5 s.
- SI prefixes (NIST): nano (n) = 10^-9, micro = 10^-6, milli (m) = 10^-3, centi (c) = 10^-2, kilo (k) = 10^3, mega (M) = 10^6, giga (G) = 10^9. So 3.5 cm = 0.035 m and 5 ms = 0.005 s.
- Change units before you calculate. Always use metres, kilograms and seconds in formulas. To change km/h to m/s, divide by 3.6: 72 km/h = 20 m/s.
- Dimensional analysis: the units on both sides of an equation must match. In s = v x t, (m/s) x s = m, so the equation is plausible. If the units do not match, the equation is wrong.
- Significant figures (OpenStax): in multiplying or dividing, your answer has as many significant figures as the input with the fewest. A circle with radius 1.2 m has area pi x 1.2^2 = 4.5239 m^2, which you report as 4.5 m^2. In adding or subtracting, keep no more decimal places than the least precise number. Zeros that only place the decimal point do not count (0.053 has two significant figures).
- Do not round in the middle. Keep extra digits on your calculator and round once, at the end.

The exercise I'm working on:
Write the five steps on a card and keep it. Solve: a runner covers 400 m in 80 s. Find the speed in m/s and km/h, then find how long 1,000 m would take at the same speed. (Answers: 5.0 m/s, 18 km/h, 200 s.)

I'm stuck. Please:
1. Explain the main idea in plain, simple words with one everyday example.
2. Walk me through the exercise one small step at a time. Ask me what I've done so far before giving the next step.
3. Don't just give me the final answer. Help me do it myself, and check my work when I share it.

I'm a parent helping my child (about 8 years old) with a lesson called "A method for every problem: units, rearranging and significant figures" from the course "Physics Problems: maths for physics".

Here's what the lesson says:
A physics problem is like a recipe. First you lay out your ingredients (the numbers you are given). Then you pick the right recipe (the equation). Then you cook it (do the maths) and taste it (ask: does this answer make sense?).

The activity:
Write the five steps on a card and keep it. Solve: a runner covers 400 m in 80 s. Find the speed in m/s and km/h, then find how long 1,000 m would take at the same speed. (Answers: 5.0 m/s, 18 km/h, 200 s.)

Please explain this so an 8-year-old understands: short sentences, a fun everyday example, and no hard words.
Then give me 3 simple questions I can ask my child to check they understood, and a way to do the activity together, one tiny step at a time.