一个盒子里面有 80 枚回形针，其中一些是灰色的，一些是黄色的，其余的是蓝色的。 摸出灰色回形针的概率是 $1/5$，摸出黄色回形针的概率是 $1/4$。

1. Find the number of grey and yellow clips, then subtract from the total.

Number of grey clips $= \dfrac{1}{5}\times 80=16$

Number of yellow clips $= \dfrac{1}{4}\times 80=20$

Number of blue clips $=80-16-20=44$

1. Express the new number of blue clips and total clips after removing $x$ blue clips.

From part (a), the original number of blue clips is $44$.

After removing $x$ blue clips,

number of blue clips $=44-x$,

total number of clips $=80-x$.

2. Set up and solve the probability equation.

$\frac{44-x}{80-x}=\frac{7}{25}$

$25(44-x)=7(80-x)$

$1100-25x=560-7x$

$540=18x$

$x=30$