Point A would be zero; Point B, the end of the 1/2-hour time interval, would be 1/2 (representing 1/2 hour). Then Amy needs to subdivide the interval A to B into three equal time intervals and to determine the length of each of these subintervals.
I would begin with 1/2 and divide that by 3:
1
--
2
====
3
--
1
Inverting the fraction in the denom. and multiplying, we get
1 1 1
-- * ---- = -----
2 3 6
So Amy will spend 1/6 th of an hour, or 10 minutes, on each chore.
Note that 3 times 1/6 comes out to 1/2 (hour), which is were we started.