3.
Three different interpretations of an 8% sales tax? Odd question.
8%, 8 percent, literally means "8 per 100". For each $100 item you purchase you pay an additional $8 in tax.
8% = 0.08, the factor applied to the price to determine the tax.
8%=8/100, you pay 8 cents in tax for every 100 cents in sales
4.
Just because the ratio is 5:7 doesn't mean the total is 5+7=12. Non zero factors don't change a ratio, e.g. 5:7=50:70.
5.
b/g=5/7
The fraction of boys is b/(b+g) which if you've done these a few times you can immediately write down as 5/12. Let's solve it like its our first time,
Similarly the fraction of girls is 7/12.