What is at the end of a rainbow?

Can you decipher the below rebus?

PBLUESAISNUERSES

One is to three as three is to five and five is to four and four is the magic number.
What is the pattern?

A man was going to bleach his socks because they had gotten muddy the day before. As he was pouring the bleach into the washing machine, he spilt some on the floor. He got some cleaning fluid and mopped it up with a rag. Minutes later he was dead. What killed him?

I am a three digit number.
My tens digit is five more than my ones digit.
My hundreds digit is eight less than my tens digit.
What number am I?

There are three boxes of different sizes with three cupcakes inside each of the boxes in Christina's kitchen. At night her daughter wakes up and goes to the kitchen. She opens a box and eats all three cakes.

But in the morning Christina finds out that each box still had three cupcakes. How?

Complete the table series riddle by finding the number that can replace "??".

I have four wings but can't fly.
I express no emotions.
I am found on the exact spot every time you look.
I keep toiling away with a little sound.

Do you know what am I?

A mathematics teacher took exams for his students. Out of the total students, 25% passed both the tests included in the exam. However, only 42% were able to clear the first test.

Can you find out the percentage of those students who passed the first test and also passed the second test?

I am a seven-letter word but if you remove four of my letters I will have one left. What word am I?