My inclination would be to be much less computer like about it. First start with some observations about the numbers. 24 is one less than 25. 4 * 25 is 100. 36

My inclination would be to be much less computer like about it. First start with some observations about the numbers. 24 is one less than 25. 4 * 25 is 100. 36 is 9 * 4. Therefor the answer is (9 * 100)-36=864. Numbers have properties. You can use these properties to do less mental work or at least take up less mental temp space. Not sure how you teach people to do that.