13 out of 25 pretzels are not checked or tested
Let the 25 pretzels be:
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25
Every fifth pretzels is tested of weight
Collect all the multiples of 5 till 25
So, pretzels that are tested for weight are: 5, 10, 15, 20, 25.
Every third pretzel is tested for size
Collect all the multiples of 3 till 25.
So pretzels that are tested for size are: 3, 6, 9, 12, 15, 18, 21, 24.
Now, pretzels that are not checked or tested are: 1, 2, 4, 7, 8, 11, 13, 14, 16, 17, 19, 22, 23.
They are 13 number.
13 out of 25 pretzels are not checked or tested.
