KA-0129Sanity-check with a second method
4 points, difficulty 2 of 3Level 5–6about 75sWork out 17 x 24, then check it a second way before you commit to an answer.
Hints
Take them one at a time. The first gives nothing away.
1A nudge
Do it once, then check it a different way.
2The strategy
Split 24 into 20 and 4 and add the two products. Then check by a different route, such as 17 times 25 minus 17, rather than reading your own working again.
3The full solution
17 x 20 = 340 and 17 x 4 = 68, giving 408. Check: 17 x 25 = 425 and 425 - 17 = 408.
Solution
The reliable way
Split the 24: 17 x 20 = 340 and 17 x 4 = 68, so the total is 340 + 68 = 408. Now check by a genuinely different route rather than re-reading: 17 x 25 is a quarter of 1700, which is 425, and 24 is one less than 25, so subtract one 17 to get 425 - 17 = 408. Two different methods agreeing is worth far more than reading the same working twice, because rereading repeats the same mistake. The transferable idea: check with a different method, not with the same one again.
The elegant way
17 x 24 is 17 x 25 minus 17, and a quarter of 1700 is 425, so the answer is 408.
Why this is on the test: Every wrong choice here is a real arithmetic slip that rereading your own working would not catch, but a second method would.