Question

Estimate a 15% tip on a dinner bill of $61.53 by first rounding the bill amount to the nearest $10

Answer 1

well, 61.53 is really 10 + 10 + 10 + 10 + 10 + 10 + 1.53.

well, 1.53 doesn't really come nowhere near close to round up to 10, so it boils down to 0, so 61.53 will just be 10 + 10 + 10 + 10 + 10 + 10 + 0 or just 60 once rounded up, now, what's 15% of 60?

$\begin{array}{|c|ll} \cline{1-1} \textit{\textit{\LARGE a}\% of \textit{\LARGE b}}\\ \cline{1-1} \\ \left( \cfrac{\textit{\LARGE a}}{100} \right)\cdot \textit{\LARGE b} \\\\ \cline{1-1} \end{array}~\hspace{5em}\stackrel{\textit{15\% of 60}}{\left( \cfrac{15}{100} \right)60}\implies 9$

well, we can get the 15% of 61.53 by just (15/100) * 61.53

however, the exercise is asking on rounding it up to "tens" first

61.53 has "10" six times plus 1.53, the rounding happens with the "surplus"

in this case the surplus is 1.53.

now, for rounding anything the tipover point is at the half, for $10 the half is $5, so if the amount is $5 or more, rounds up to $10, if less than that goes to 0.

1.53 is nowhere near 5, which is the halfpoint or tipover point, because of that, it rounds up to 0, so 61.53 rounded up to "tens" only will be "six tens" only, or 60.