4th grade
posted by dk .
reena says the product of 54 and 9 cannot be 487. she says that she knows this without actually multiplying. how does she know?

what is the last digit of 4x9?
4x9=36, so last digit is 6, which doesnt match last digit in 487.
Another way. The nines divisibility rule: 9 is evenly disible in any number in which the sum of digits is 9. Adding 4,8,7 gives 19, and 9 wont go into that. 
54 * 9 cannot be 487 because 4 * 9 = 36. The ones digit must be 6.