Library / /48 of 241

, 2020

Algebra Difficulty 1.4 Junior Find the answer Canada

The sum of the first 9 positive integers is 45; in other words, $1+2+3+4+5+6+7+8+9=45$ What is the sum of the first 9 positive multiples of 5? In other words, what is the value of $5+10+15+\cdots + 40+45$ ?

$225$
$250$
$180$
$150$
$450$

Your answer

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Since $1+2+3+4+5+6+7+8+9 = 45$ then $5+10+15+ \cdots + 40 + 45 = 5(1+2+3+\cdots + 8 + 9) = 5 (45) = 225$

Want a route through all this instead of an archive? The track puts 2,444 problems in a working order, from Junior Challenge level to the IMO shortlist.

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.