Question

if you get paid $9.99 for every sock and make 390 socks, estimate how much will you be paid.

Explanation:

Step1: Round the pay per sock

To estimate, we can round $\$0.99$ to $\$1$ (since $0.99$ is very close to $1$).

Step2: Multiply by the number of socks

We have 390 socks. So we calculate $1\times390 = 390$. But since we rounded up the pay per sock a little (from $0.99$ to $1$), a more accurate estimate considering the slight difference: originally, the pay is $0.99\times390$. We can also think of $0.99 = 1 - 0.01$, so $0.99\times390=(1 - 0.01)\times390 = 390- 3.9 = 386.1$. But for a simple estimate, using the rounded value of $1$ per sock gives us approximately $\$390$, and the more precise estimate (since $0.99$ is almost $1$) is very close to $\$390$ (actually $\$386.1$, but when estimating, we can say around $\$390$ or more accurately, since $0.99$ is 1 cent less than 1, so total pay is $390 - 3.9 = 386.1$, so approximately $\$386$ or $\$390$ depending on the level of estimation. But the most straightforward estimate is to use $1\times390 = 390$.

Answer:

Approximately $\$390$ (or more precisely around $\$386.10$)