Maths Olympiad Prep

Library / /133 of 144

Number theory Difficulty 2.7 Junior Find the answer

A two-digit positive integer xx has the property that when 109 is divided by xx, the remainder is 4. What is the sum of all such two-digit positive integers xx?

A number or a short expression. Spacing and $ signs are ignored.

Solution

Suppose that the quotient of the division of 109 by xx is qq. Since the remainder is 4, this is equivalent to 109=qx+4109=q x+4 or qx=105q x=105. Put another way, xx must be a positive integer divisor of 105. Since 105=5imes21=5imes3imes7105=5 imes 21=5 imes 3 imes 7, its positive integer divisors are 1,3,5,7,15,21,35,1051,3,5,7,15,21,35,105. Of these, 15,21 and 35 are two-digit positive integers so are the possible values of xx. The sum of these values is 15+21+35=7115+21+35=71.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.