Question

Dado el número N = 4752a, averigua qué valor ha de tomar "a" para que: a) N sea divisible por tres. b) N sea divisible por cinco. c) N sea divisible por 15. d) Cuando se divida N por 7 dé de resto 5.

Answer 1

I will answer it in English:

a) We have te number N = 4752a

if we want to divide it by 3, then the addition of all the digits must be a multiple of 3.

Sum = 4 + 7 + 5 + 2 + a = 6 + 7 + 5 + a = 18 + a

and 18 is a multiple of 3, then we can have, a = 0, a = 3, a = 6, a= 9

b) A number is divisible by 5 if it ends on 5 or 0, so the possible values of a are 0 and 5.

c) we know that 15*3 = 45

then 15*3000= 45000

now we have:

4752a - 45000 = 252a

15*13 = 2520

then we have that a must be equal to zero, and:

15*(13 + 3000) = 47520

d) We want a number that when we divide it by 7, the surpass is 5.

a number is divisible by 7 if the difference between the number without the units and the double of the units is 0 or a multiple of 7.

in N = 4752x the unit is a, then we have:

D = 4752 - 2*x

now, let's choose a such we have a multiple of 7

we know that 5746 is a multiple of 7, so we can choose x= 3 and get:

D = 4752 - 2*3 = 5746

So the number N = 47523 is a multiple of 7, and when we divide it by 7 we will have a surpass of 0.

Then when we divide N + 5 by 7, we will have a surpass of 7.

This is: N + 5 = 47528

This means that the actual value of a that we need is 8.