Between 1986 and 1989, the currency of our country was the cruzado ( $\mathrm{Cz} \$$ ). Since then, we have had the new cruzado, the cruzeiro, the new cruzeiro, and today, we have the real. To compare values from the time of the cruzado and today, economists calculated that 1 real is equivalent to 2750000000 cruzados. Imagine that the currency had not changed and that João, who earns 640 reais per month today, had to receive his salary in 1 cruzado notes only. If a stack of one hundred 1 cruzado notes measures 1.5 cm in height, what would be the height (in kilometers) of João's salary?
(a) 26.4
(b) 264
(c) 26400
(d) 264000
(e) 2640000
The correct option is (d).

The statement says that 1 real $=275 \times 10^{7}$ cruzados. João's salary is 640 reais, which is equivalent to $640 \times 275 \times 10^{7}=176000 \times 10^{7}=176 \times 10^{10}$ cruzados. The number of stacks of one hundred notes that can be made with this amount of 1 cruzado notes is $176 \times 10^{10} / 10^{2}=176 \times 10^{8}$. Since each of these stacks is $1.5 \mathrm{~cm}$ high, the total height of all of them is $1.5 \times 176 \times 10^{8}=264 \times 10^{8} \mathrm{~cm}$.

Now we remember that $1 \mathrm{~km}=1000 \mathrm{~m}=10^{3} \mathrm{~m}$ and that $1 \mathrm{~m}=100 \mathrm{~cm}=10^{2} \mathrm{~cm}$, so $1 \mathrm{~km}=10^{3} \times 10^{2}=10^{5} \mathrm{~cm}$. Thus, a stack of $264 \times 10^{8} \mathrm{~cm}$ has $264 \times 10^{8} / 10^{5}=$ $264 \times 10^{3}=264000 \mathrm{~km}$ in height.