hey I'm Ashley and today we're going to practice adding unknown numbers or
finding the missing number in an addition problem are you ready let's go
so when we are looking for an unknown number it's really important to
understand the concept of fact families look here at this fact family I have
three plus two equals five two plus three equals five and then take the sum
move it over to the first number and a subtraction problem five minus three
equals two and five minus two equals three so how can I use this to help me
solve a problem with an unknown number let's see let's look at if we have the
problem four - an unknown sorry plus an unknown equals six how would I do this
well remember just like over here I can take this sum and make it the first
number of a subtraction problem so six minus and then I take one of my numbers
here which I only know four at the moment six minus four is equal to well
if you have these memorize you know six take away four is two so now if six
minus four equals two I know that four plus two is equal to six
now let's go try this with some of the problems from our unknown numbers games
okay here I have the problem nine plus an unknown equals 10 so remember I want
to take this sum here 10 and then I'm going to subtract the number I do know
nine and that equals 10 minus 9 equals one so now if I was to take that and put
it in there nine plus one equals ten here's our
answer one okay this next one is a bit trickier I bet you do not have this fact
family memorized yet do you don't worry we can still solve it okay we're going
to use the same process I'm going to take my sum of 40 and I'm going to
subtract 28 now I will write this vertically this time it makes it easier
to subtract when I have my place values lined up okay zero minus eight can we do
that no we need to borrow if you don't know
borrowing yet be sure you go back and watch our videos on subtraction with
borrowing or regrouping okay so I'm going to borrow from this four in the
tens place I'll take a ten from the four so that makes the four a three
I can add that 10 to the zero so it makes it ten now I can subtract Tad's
minus 8 equals two good and then three minus two is equal
to one so 12 is our final answer 28 plus 12 is equal to 40 let's try one
more
whoo look at this one these numbers are big we can do it though same way let's
take our sum 308 and subtract our first number here 299 and let's see what we
get can we subtract from the 8 no we need to
borrow but we can't take any tents there are none so let's come over to the
hundreds we will take a hundred from this 3 so that makes only two hundreds
there now if we add that to the tens place we have ten hundreds or sorry ten
tens and now we can borrow from the tens
place take 1/10 away so there's nine tens and
this eight can become an 18 we added a 10 there so now 18 minus 9 is 9 good 9
minus 9 is 0 and 2 minus 2 is 0 so I don't need to put any zeros there I just
have one digit in the ones place 9 so that means our answer 299 plus nine is
308 and there is our answer 9 and there you have it you just learned
how to find an unknown number in an addition problem now go practice what
you've learned with our online games and quizzes and remember to always be clever