First you have to find what 5% (sales tax) of 20 is:
[tex] x_{1} = (5 \div 100) \times 20 \\
x_{1} = 100 \div 100 \\ x_{1}= 1[/tex]
Then you have to find what 15% (tip) of 20 is:
[tex] x_{2} = (15 \div 100) \times 20 \\ x_{2} = 300 \div 100 \\ x_{2} = 3[/tex]
So now you can calcute how much money you can spend:
[tex]x = 20 - x{1} - x{2} \\ x = 20 - 1 - 3 \\ x = 16[/tex]
