Question

How many numbers between 40 and 129 are equal to 6 times an integer

Answer 1

Here, the series would be: 42, 48 .... 126
an = a + (n - 1)d
126 = 42 + (n - 1)6
126 - 42 = 6n - 6
84 + 6 = 6n
n = 90/6
n = 15

In short, Your Answer would be 15

Hope this helps!