468 take away nine equals what?
Here we’re being asked to take away a single-digit number from a three-digit number. So we start with 468. 468 has eight ones and not nine ones. How can we partition the number nine to help us subtract it quickly?
We can split it into eight and one. Why have we done this? Why not five and four or six and three? Well, because we can subtract eight from 468 quickly. 468 take away eight leaves us with 460. Now we just need to subtract the remaining one. 460 take away one equals 459.
Another mental method that we could use that would give us exactly the same answer, nine is very close to 10 and 10 is a number that we can subtract really quickly. What if we subtracted 10 and then adjusted our answer?
Start with 468 and then take away 10, which is something we can do really quickly. 468 minus 10 equals 458. But the question doesn’t ask us to subtract 10. It asked us to subtract nine. We’ve taken away one too many. Because we’ve taken away one too many, we need to add one to our answer to adjust it. 458 plus one equals 459.
Two different methods, but they’re both ways that we could find the answer mentally. In the first method, we notice that 468 had eight ones. And we knew that we could subtract eight quickly. So we partition nine into eight and one. We subtracted eight and then the one to give us the answer 459.
In our second method, we notice that the number nine is very close to 10. And we knew that we could subtract 10 quickly. So instead of taking away nine, we took away 10. But because we’d subtracted one too many, we added one to adjust our answer. Subtracting 10 then adding one is the same as subtracting nine. 468 take away nine equals 459.