One thing to recognise is after you add the new number of $1 coins to your collection, the number of other coins remains the same.

Previously,
Number of $1 coins = 200 x 25% = 50 coins
Number of other coins = 200 x 75% = 150 coins

After adding more $1 coins,
$1 will then take up 40% of total coins, while other coins take up 60%

60% = 150 coins
1% = 150 ÷ 60
40% = (150÷60) x 40
= 100 coins

There's 100 $1 coins now.
So, number of $1 coins to add is 100 - 50 = 50 coins.