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Early math review

Course: Early math review>Unit 5

Lesson 11: Addition and subtraction missing value problems

Missing numbers in addition and subtraction

Sal solves missing number in problems like 73 = ___ + 57.

Want to join the conversation?

• To answer some questions in which we have to find the missing numbers by addition and subtraction within 100, we just have to use inverse operations, right?
• Yes. So if you have for instance the problem: 45 equals _ minus 13, then you know that it is the same as 45 plus 13, which equals 58. Have you done any algebra yet? This is pretty much very basic algebra.
• 41 = 79 - _..... I would think that I could just do my work as 79 + 41 = 120 - 79 = 41 correct? When I do the practice it is telling my that is incorrect. I look at the hint and I understand the reducing the number but the answer is correct either way. Is that just how the system is set up?
• 79 + 41 = 120
and 120 - 79 = 41
but in your quest its '79 - _' not '_ - 79'
you must be thinking they both are the same but no...
if you subtract a smaller no. from greater no. ( 120 - 79 ) you will get a positive no. (41)
but, if you subtract a greater no. from smaller no. (79 - 120) you will get a negative no. (- 41)
now, lets get back to your question...
41 = 79 - _
we can write it as
_
= 79 - 41..........[ here i took 41 from left hand side (LHS) to right hand side (RHS) and when we do this the sign of any no. changes as here 41 was positive after taking it other side it became negative and '_' was negative after taking it to LHS from RHS it became positive]
now, _
= 38
hope you got that