how many 4-digit numbers are there such that the thousands digit is equal to the sum of the other three digits?
4-digit-number:~abcd=a103+b102+c10+da=b+c+d⇒(b+c+d)103+bcd10<10000⇒(b+c+d)103<10000−bcd10⇒(b+c+d)<10000−bcd10103⇒(b+c+d)<10−bcd101000⏟0…0.999⇒(b+c+d)<10
b+c+d<10(b=0)cd000001002003004005006007008009010011012013014015016017018020021022023024025026027030031032033034035036040041042043044045050051052053054060061062063070071072080081090sumb=0=1+2+3+4+⋯+10
bsum01+2+3+4+5+6+7+8+9+10=5511+2+3+4+5+6+7+8+9=4521+2+3+4+5+6+7+8=3631+2+3+4+5+6+7=2841+2+3+4+5+6=2151+2+3+4+5=1561+2+3+4=1071+2+3=681+2=391=1sum=220
There are 220 - (abcd = 0000) = 219 4-digit numbers
Hallo Melody, i think add for the first digit 5 the following digits 221 with 3 numbers of ways and you get
216+3 also 219 4-digit numbers
