Quick questions on Blood alcohol content (BAC) for HSC Maths Standard 2

The method is just substitution into a formula, the same skill you use for any other formula on the course. First choose the male or female version to match the person. Then replace $N$, $H$ and $M$ with their numbers. Work out the top and the bottom separately before you divide, because the top is a subtraction and the bottom is a multiplication.

A BAC table saves the substitution by listing the estimate for common combinations of body weight and number of drinks, usually for one hour of drinking. The two inputs are the column (body weight in kilograms) and the row (number of standard drinks); the value where they meet is the BAC. To read it, find the weight column closest to the person, run down to the drinks row, and take the cell value. The table below is built from the male formula with $H = 1$, and the highlighted cell shows that an $80$ kg male who has $4$ standard drinks in an hour is estimated at a BAC of $0.060$.

The body removes alcohol at a roughly steady rate of about $0.015$ of BAC per hour, no matter how high the BAC started. Because the fall is steady, the time for the BAC to drop back to zero is the starting BAC divided by that rate:

The wait you calculate only means something against a limit, and in NSW there are three:

The "hours to wait" calculation is short, but the marks reward laying it out cleanly and choosing the right target. The four stages below take Liam's BAC of $0.065$ from the worked example through to the wait for a full-licence driver, who needs the BAC down to $0.05$, not zero. The key decision is which drop to divide by $0.015$: the whole BAC for a zero limit, or just the part above the limit for a full licence.