Question

How many numbers between 1 and 400 are divisable by 7

Answer 1

400/7 = 57.14...
57 * 7 = 399

{7, 14, 21 .... 399}


`a_1=7\\\\ a_n=399\\\\ d=7\\\\ a_n=a_1+(n-1)d\\\\ 399 = 7 +(n-1)7 \ |:7\\\\ 57=1+n-1 \\\\n = 57`

Answer 2

divide 400 by 7. You get 1 as a remainder. now subtract one from 400 to get the greatest number less than 400 which is divisible by 7.
Now consider multiples of 7*1=7 is the 1st
similarly 7*2=14 is the 2nd
if their are n numbers than 7*n=399
u get n =57 which is the answer