2/5 of her money on 12 identical pencils

2 units --> 12 pencils
1 unit --> 12/2 = 6 pencils ("/" is a symbol for divide, and in fact, fraction signifies division!)
5 units --> 6 x 5 = 30 pencils

So Raihan has just enough money to buy 30 of such pencils.

15 of such pencils would have cost Raihan 15/30 = 1/2 (divide both numerator and denominator by the common term 15) of her money.

Since half of her money was spent on the pencil, the amount of money spent on the pens was half (1 - 1/2 = 1/2).

Sean had 50 twenty-cent coins.

2 units --> 50 coins
1 unit --> 50/2 = 25 coins
3 units --> 25 x 3 = 75 coins

So Claire had 75 twenty-cent coins.

Total value of Claire's coins
= 75 x 20
= 1500 cents
= $15

so Claire had $15.

Thank you very much to help solve my problem.