standard_description: Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
problem_activity: Explain why
$${\frac{1}{2}ab\mathrm{sin}(C)}$$
gives the area of a triangle with sides
$$a$$
and
$$b$$
and included angle
$$C$$
.


aligns: no