hi I'm Ashley and welcome to subtracting unknown numbers in this video we're
going to practice finding an unknown number in subtraction problems you ready
let's go
like with adding unknown numbers we can use fact families to help us find the
missing number in a subtraction problem as well so look at this example fact
family here on the Left I have three plus two equals five two plus three
equals five then the sum becomes the first problem
of the last two subtraction problems or the first number sorry five minus three
equals two and five minus two equals three notice that in these two problems
the numbers just switch places this is what will help us let me show you what I
mean so if I had a different problem let's
say six minus an unknown is equal to four
just like over here five minus three equals two the two switched places by
minus two equals three so then the three came over here so I can do the same
thing I'll do another subtraction problem 6 - let me bring my four down
six minus four is equal to what this is the answer six minus four equals two so
that means six minus two is equal to four let's try this with some of our
problems from the game see here I have five minus a number equals three so if I
just move that three over then I will get the other problem in our fact family
and this will help us find the unknown number let's say I have five and I take
away three what do I get five minus three equals
- and there is our answer let's try another one a little bit more
challenging ok my next one here is 46 - a number equals 9 let's just move the 9
over and take it away from 46 and see what we get 46 minus 9 equals notice I'm
subtracting vertically so I can keep my numbers or my place values lined up
because I will need to borrow and this is very helpful
so I cannot take 9 away from 6 because 9 is bigger so I will have to borrow from
the tens place take one turn away that means I have three tens add that 10 to
the sixth makes 16 so 16 minus 9 is 7 now three - nothing is three look over
here here's our answer 37 now if we wanted we could go back and check our
work by plugging in this 37 into the problem so let's try that so plugging it
in I have 46 and I'm going to subtract 37 and see if we get 9 okay we cannot
take 7 from 6 so we have to borrow the 4 becomes a 3 6 becomes 16 16 minus 7
equals 9 and 3 minus 3 is 0 so our final answer 9 so that means 46 minus 37
equals 9 and we confirmed that by checking our work all right so with this
next one we have 103 minus an unknown equals 67 let's flip that problem around
so we have 103 and let's subtract 67 7 from 3 we'll
need to borrow which we can not because there are no tens to borrow from so
we'll need to borrow from the hundreds we now have zero hundreds which makes 10
tens now we can borrow from the tons place so we have nine tons that makes
this 3 a 13 so 13 minus 7 6 9 minus 6 is 3 0 minus 0 is nothing so we have
nothing in the hundreds place our answer is 36 103 minus 36 equals 67 I'm going
to work through one last problem here this comes from our subtracting unknown
numbers to game and it is a bit more advanced it does include negative
numbers so if you have not learned those yet don't worry about it don't go to
that game yet just stick with unknown numbers 1 and when you're ready and you
have learned negatives then you are ready to start subtracting unknown
numbers - all right so let's look at this now I'm going to do this the same
way I will take my difference here move it over so that I'm going to now have 28
minus a negative 90 equals what well if you remember when you subtract a
negative the subtraction and negative sign go together to make a positive or a
plus so now I have 28 plus 90 let me rewrite this vertically 28 plus 90 is
equal to what well 8 plus 0 is 8 9 + 2 is 11 so my final answer is 108 teen and
that's it be sure you go practice what you've learned with our online games and
quizzes and remember to always be clever