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### Course: Class 1>Unit 2

Lesson 1: Counting numbers from 1 to 20

# Intro to place value

Sal uses the number 37 to explain why we use a "ones place" and a "tens place" when writing numbers.  Created by Sal Khan.

## Want to join the conversation?

• If you just wanted to group scratches of ten together, wouldn't you leave the ones still as scratches but just change the tens to something else? Maybe an X? So it'd be `XXXIIIIIII` for thirty-seven?
• Yes, and that's essentially what Sal did, but using our already known number notation. We could call `XXXIIIIIII´: 3 X and 7 I. However, we know X represents ten I, so we call it "ten". If we have three of those tens, we join the words in a funny way and call it "thirty", then we are still left with 7 ones, which we just call seven. To join our thirty and seven we just call thirty seven (37).
• How could you become really proficient in place value?
• Practice and repetition will help a lot.
If you understand the idea of place value, you will succeed!
• Why doesn't Sal put the dash in the 7?
• 7's can often be confused with other number in many people's handwriting. Putting a dash in there makes it look less like a 1.
• Does numerals mean numbers?
• 1=I 2=II 3=III 4=IV 5=V 6=VI 7=VII 8=VIII 9=IX 10=X

• Are there place values in decimals?
• yes there are tenths hundredths thousandths then hundreds tens and ones
e.g 103.54= 1 thousandths 0 hundredths 3 tenths. 5 tens 4 ones
• Is there a hundreds place and so on?
• Yes, in fact, there are infinity(goes on forever) places. Since numbers do not end, place values do not end. I hope this helped :)
• We could have any unit to describe time then? Like a month could be a multiple of ten or 24?
• Yes. Time can be measured in multiples of seconds, minutes, hours, days, weeks, months, years, decades, centuries, millenniums . . . ok you get the idea :)
• What sign is used for greater, less, equal?
• < means "less than". So, 3 < 5 is read as 3 is "less than" 5
> means "greater than". So 6 > 2 is read as 6 is "greater than" 2
= means "is equal to". So 1 + 1 = 2 is read as 1 + 1 "is equal to" 2
• Is it valid to say that when it comes to whole-number place value, the number value extends to the left but when dealing with decimals, the values extend from left-to-right?