Bunuel
Find the sum of all the 3-digit positive integers which are divisible by 3, but not by 19.
A. 156,372
B. 155,517
C. 156,486
D. 155,487
E. 156,283
1st \(3\) digit divisible by \(3 =3*34 = 102\)
Last \(3\) digit divisible by \(3 =3*333= 999\)
\(333-34+1 = 300 \)= Total number of three digit numbers divisible by three.
Hence sum of all three-digit positive integers which are divisible by \(3\)
\(\frac{300}{2} (102+999 )= 165,150\) : using sum of \(n\) terms in AP \(= \frac{n}{2}\)( 1st term + last term )
Number of terms divisible by \(57 \)( Terms need to be divisible by both \(19\) and \(3\) hence \(19*3 =57\) )
1st three digit number divisible by \(57 =57*2= 114\)
Last three digit number divisible by \(57 =57*17= 969\)
\(17-2+1 = 16 \): Total number of three digit numbers divisible by \(57\)
Hence sum of all three-digit positive integers which are divisible by \(57\)
\(\frac{16}{2} (114+969)= 8664\)
Therefore sum of all the 3-digit positive integers which are divisible by \(3\), but not by \(19.\)
\(165,150 - 8664 = 156,486\)
Ans C
Hope it helped.