甲是40，乙是甲的5分之4，那么我们要计算出乙的具体数值。首先，我们可以将甲的数值代入到公式中，即乙 = 甲 × 5/4。接下来，我们将40代入公式中进行计算：乙 = 40 × 5/4 = 50。

Paragraph 1: Introduction

甲是40，乙是甲的5分之4。这个问题需要通过代入公式进行求解。通过简单的计算，我们可以得出乙的数值为50。本文将探讨这个问题的背景和计算过程。

Paragraph 1: Introduction (Translation)

甲是40，乙是甲的5分之4。(Jiǎ shì sìshí, yǐ shì jiǎ de wǔ fēn zhī sì.) This problem requires solving through the substitution of values into a formula. By simple calculation, we can determine that the value of yǐ is 50. This article will explore the background and calculation process of this problem.

Paragraph 2: Understanding the Problem

在解决问题之前，让我们更深入地理解题目中的关键词。首先，我们知道甲的值是40。其次，我们知道乙是甲的5分之4。这意味着乙的数值应该比甲的数值大，因为5分之4大于1。

Paragraph 2: Understanding the Problem (Translation)

Before solving the problem, let us have a deeper understanding of the key terms in the question. First, we know that the value of jiǎ is 40. Second, we know that yǐ is 5/4 of jiǎ. This means that the value of yǐ should be larger than the value of jiǎ since 5/4 is greater than 1.

Paragraph 3: Calculation

根据题目中给出的公式，我们可以代入甲的值进行计算。将40代入公式，我们有乙 = 40 × 5/4 = 50。因此，乙的数值为50。

Paragraph 3: Calculation (Translation)

According to the formula given in the question, we can substitute the value of jiǎ into it for calculation. By substituting 40 into the formula, we have yǐ = 40 × 5/4 = 50. Therefore, the value of yǐ is 50.

Paragraph 4: Verification

为了验证我们的计算是否正确，我们可以将乙的数值代入公式进行反推。根据题目中的公式，甲应等于乙的4/5。将50代入公式，我们有甲 = 50 × 4/5 = 40。这与题目中给定的甲的值相符，所以我们的计算是正确的。

Paragraph 4: Verification (Translation)

To verify if our calculation is correct, we can substitute the value of yǐ back into the formula. According to the formula given in the question, jiǎ should be equal to 4/5 of yǐ. By substituting 50 into the formula, we have jiǎ = 50 × 4/5 = 40. This matches the given value of jiǎ in the question, so our calculation is correct.

Paragraph 5: Conclusion

综上所述，乙的数值为50。通过代入甲的值，并根据题目中的公式进行计算，我们得出了这个结果。这个问题提醒我们要仔细阅读题目，理解并运用相关的数学知识来解决问题。

Paragraph 5: Conclusion (Translation)

In conclusion, the value of yǐ is 50. By substituting the value of jiǎ and using the formula given in the question, we have obtained this result. This problem reminds us to read the question carefully, understand it, and apply relevant mathematical knowledge to solve problems.