I like HK's comments, and will also add:
- you're already phrasing it as "reasonableness check" (rather than just as "check your work") which I think is a win; it's usually more effective to do something *different* from just rerunning the process which is what often seems to be meant by "check". Easy numbers to rerun the process on is one idea. Another might be: by adjusting the numbers, come up with something that's definitely too big and something definitely too small to be the right answer. (e.g. if the problem is to find 17.5% of 420, well, that's got to be bigger than 10% of 420 which is 42, and smaller than 20% of it which is 84). Check that the actual answer you get is between the two.
- If you think she'd be up for it, how about some dimensional analysis? (One of those things I think should be introduced, ooh, about 10 years earlier in the educational process than it typically is.) E.g., if you're asked to find out how long something takes in s from data that includes a distance in km and a speed in km/h, multiplying the distance by the time is not reasonable because the resulting units would be km^2/h which is not a measure of time, while dividing the distance by the speed is reasonable because the resulting unit is km/ (km/h) = h which is a unit of time.
(One of DS's commonest problems, btw, would be to write down the answer in hours instead of the requested seconds - back to point 1!)
