john wants to send a letter to peter

Answer:

The minimum number of letters John has to send to be sure that Peter receives his letter is 127 letters

Step-by-step explanation:

The four digit numbers that are multiples of 5 and 7 with the last digit = 0 is found as follows

Since the last digit of the house number = 10, then the house number is divisible by 10 which also meets the condition that the house number is divisible by 5

We have the four digit numbers from 1000 to 9999

Hence the numbers divisible by both 7 and 10 are from (1000/70 (Which is 14 + 2/7) - 2/7)×70 + 70 = 1050 to (9999/70 (Which is 142 + 59/70)- 59/70)×70= 9940

Which gives 142 - 15 = 127 numbers which are four digit number multiples of 5 and 7 with the last digit = 0

Hence the minimum number of letters John has to send to be sure that Peter receives his letter = 127 letters.