sum of multiples of 3 under 553

553 -1 =552

552/3 =184 terms

Sum of 184 "3's" =51,060.

553 - 3=550

550/5 =110 terms

Sum of 110 "5's" =30,525

$553\div5=110.6$

 there is 110 numbers wich is devisible by 5

$553\div3=184.333$

there is 184 numbers wich is devisible by 3

$553\div15=36.9$

there is 36 numbers wich is devisible by both 5 and 3

now let s find sum of devisors of 5:

$5+5\times2+5\times3+5\times4\dots+5\times110\\ 5(1+2+3+4\dots+110)\\ 5(55.5\times110)=30\ 525$

sum of devisors of 3:

$3(92.5\times184)=51\ 060$

sum of devisors of both 5 and 3

$15(18.5\times36)=9990$

$30\ 525+51\ 060-9990=71\ 595$