Question

 find the largest five digit number is exactly divisible by 7, 10, 15, 21 and 28
  A)99840   B)99900    C)99960    D)99990

Answer 1

LCM of 7,10,15,21, and 28 
7 is prime
10 = 2*5 
15 = 3*5
21 = 3*7
28 = 2²*7

LCM = 2² × 3 × 5 × 7 = 420
So the number should be divisible by 420

Largest five digit number = 99999

99999 ÷ 420 = 238$\frac{39}{420}$

So remainder is 39.
The largest number divisible = 99999 - 39 = 99960

Answer 2

99960 is the correct answer