Answer :
We are given the following information.
Santiago gets paid $3 per hour.
He worked on both Saturday and Sunday and made a total of $21.
Santiago worked for 4 hours on Saturday.
We are asked to find out how many hours did he work on Sunday?
On Saturday, he worked for 4 hours.
[tex]Sat=\$3\times4=\$12[/tex]So, he got paid $12 on Saturday.
Let x be the number of hours he worked on Sunday.
Then we can set up the following equation
[tex]12+3\cdot x=21[/tex]Where 3x represents the money he got on Sunday.
The sum of the money for Saturday and Sunday must be equal to the total amount of $21.
Let us solve for x.
[tex]\begin{gathered} 12+3x=21 \\ 3x=21-12 \\ 3x=9 \\ x=\frac{9}{3} \\ x=3 \end{gathered}[/tex]This means Santiago worked 3 hours on Sunday.