16⅔ % = 16⅔ / 100 = 50/3 / 100

= 50/3 × 1/100

= 1/3 × 1/2

= 1/6


Or

100 ÷ 6 = 16⅔

So 16⅔ out of 100 is equal to 1/6.

Let the number of foreign stamps at first be 6u.

After giving away 5/6 of them to friends, remaining foreign stamps = 6u - 5u = 1u

After buying 28 stamps, he has (1u + 28) foreign stamps .


This is 1/6 of local stamps left.


So local stamps left

= 6 × (1u + 28)
= 6u + 168


Now, 40% = 40/100 = 2/5

1 - 2/5 = 3/5

So 3/5 of the local stamps are left. And that is equal to 6u + 168 stamps.


(6u + 168) ÷ 3 = 2u + 56
(2u + 56) × 5 = 10u + 280

So number of local stamps at first = 10u + 280

Total number of stamps at first
= 6u + 10u + 280
= 16u + 280


So 16u + 280 = 424

16u = 424 - 280 = 144
1u = 144 ÷ 16 = 9

Number of local stamps at first
= 10u + 280
= 10 × 9 + 280
= 90 + 280
= 370

OR

Number of local stamps at first
= Total - foreign
= 424 - 6u
= 424 - 6 × 9
= 424 - 54
= 370