total days → 210 days
total coins from susan → 210 x 2 = 420
every 5 day → 1 coin added
number of coin from father in 210 days → 210 ÷ 5 = 42
total coins collected → 420 + 42 = 462
total value of $1 coin → 42 x $1 = $42
value without $1 coin → 225 - 42 = $183
total number of coins without $1 coin → 210 x 2 = 420
assumption method
step 1 → assume all coins to be 20¢ coins
total value if all 20¢ coins → 420 x $0.20 = $84
step 2 → difference for value and total
total difference → $183 - $84 = $99
value difference → $0.50 - $0.20 = $0.30
step 3 → divide
number of 50 cent coins → $99 ÷ $0.30 = 330