Answer
a) $\approx 0.05\;\rm N$
b) $\approx 35\;\rm N$
Work Step by Step
a) We need to estimate the weight of a pencil by using the information given in Table 5.1, as the author told us.
We are given 3 weights of objects in the table, a quarter, a pound, and a person.
The closest weight to our pencil computer is the weight of a quarter which weighs 0.05 newtons.
We can guess that the pencil's weight is roughly equal to (or less than) the weight of one-quarter, so the weight of the pencil is
$$W_{pencil}\approx \bf \color{red}{0.05}\;\rm N$$
b) We need to estimate the propulsion force of a sprinter by using the information given in Table 5.1, as the author told us.
We are given a car's propulsion force, which is about 5000 N.
An average car is about 1000 kg while a sprinter is about 70 kg, as a rough estimate.
Now we can assume that the acceleration of the sprinter is one-tenth that of the car, so the propulsion of the sprinter is given by
$$\dfrac{F_{sprinter }}{F_{car}}=\dfrac{m_{sprinter }a_{sprinter }}{m_{car}a_{car}}=\dfrac{70\cdot 0.1a_{car}}{1000a_{car}}=\dfrac{7}{1000}$$
Thus,
$$ {F_{sprinter }} =\dfrac{7}{1000}F_{car}=\dfrac{7}{1000}\cdot 5000\;\rm N =\color{red}{\bf 35}\;\rm N$$
