Stella earn by babysitting is $5x and earn by working at an shop of icecream is $8y.
Part (A)
Stella earn at least $120 means sum of Stella's earning by babysitting and by working at an shop of ice cream is more than or equal to 120. So equation is,
[tex]5x+8y\ge120[/tex]Stella cannot work more than 20 hours. So equation is,
[tex]x+y\leq20[/tex]Part (B)
Substitute 7 for x in the equation
[tex]5x+8y\ge120[/tex]to obtain the value of y.
[tex]\begin{gathered} 5\cdot7+8y\ge120 \\ 35+8y-35\ge120-35 \\ \frac{8y}{8}\ge\frac{85}{8} \\ y\ge10.62 \end{gathered}[/tex]So value of y is greater than or equal to 10.62. So nearest whole number for y is 11.
Thus Stella work at least for 11 hours at the ice cream for eaning to be at least $120.