The second stanza of the poem is difficult because it is a cultural reference to an old piece of traditional English literature. It is still possible to understand it if you read it carefully. This is mostly a language puzzle, not a math puzzle. And the author did not mean for it to be puzzling. There is nothing tricky here, except for literary language and cultural reference.
You probably will need the clue I've included, after the poem.
It is from A. E. Housman's series of poems, "A Shropshire Lad." It was published in 1896. The poems are untitled. They are referred to by their first line. "Lad" means a young man. "Shropshire" is a county in England.
First, the poem, "Loveliest of trees, the cherry now."
Loveliest of trees, the cherry now
Is hung with bloom along the bough,
And stands about the woodland ride
Wearing white for Eastertide.
Now, of my threescore years and ten,
Twenty will not come again,
And take from seventy springs a score,
It only leaves me fifty more.
And since to look at things in bloom
Fifty springs are little room,
About the woodlands I will go
To see the cherry hung with snow.
The question: how old is the person speaking in the poem?
Notes: "A score" can mean twenty. It does so here. Thus "threescore years and ten" means "seventy." This use of the word "score" is very old-fashioned. When we see "score," we know that the author is quoting centuries-old English, or is trying to sound old-fashioned.
To answer the question, don't jump to a conclusions. Read all of the parts of the poem that mention numbers: "threescore years and ten" (= seventy), "twenty," "fifty," "a score" in the second stanza, and "fifty" again in the last stanza.
A clue: to see it, "select all" or drag mouse from here
"Threescore years and ten" is a traditional value for the human lifespan. "My threescore years and ten" means that the speaker feels entitled to live to age seventy, he "has" a total of seventy years of life all together.
The writer is quoting the most famous English translation of Psalm 90, from the Old Testament: "The days of our years are threescore years and ten."
Yes, you are correct. He is twenty years old.
He "has" a lifespan of seventy years.
Of his seventy years of expected life, "twenty will not come again," they are the twenty years he has already lived.
"Take from seventy springs a score" means that in his life, he will see the season of spring seventy times. If you subtract the score of springs--the twenty springs--he has already lived through, he has only fifty more springs ahead of him.
Looking at things blooming in spring is so wonderful that even fifty of them isn't enough--"Fifty springs are little room." He doesn't want to waste them, so he is going to hurry and look at the cherry trees.
A clear, simple explanation for such a great puzzle, I have to say.
At my first try, I subtracted another twenty when he stated: "Take from seventy springs a score", and I thought it was strange that he later said: "It only leaves me fifty more", so I was sure my answer is reasonably wrong.