Answer:
90 red pens
Explanation:
we need to find the least common multiple of the numbers 3 , 5 , 6 , 9
6 = 2 × 3
9 = 3²
then
LCM(3,5,6,9) = 2×5×3² = 2×5×9 = 90
The number 90 can be divided into 3 ,5 ,6 ,9 with no remainder
Therefore the pack must have 90 pens or a multiple of 90
For example 180 , 270 ...