一題數 , 但需要有詳盡解釋

設本金為y.

y(6/100)(21/12) - y(4.75/100)(21/12) = 787.5
0.105y - 0.083125y = 787.5
0.021875y = 787.5
y = 36000

Proof:
年利率6%: $36000x6/100x21/12 = $3780
年利率4.75%: $36000x4.75/100x21/12 = $2992.5
相差: $3780 - $2992.5 = $787.5

[因為是年利率, 所以21月要轉成21/12(年)]