There are 121pupil in a computer class 8/9 of the boys and 4/5 of the girls were African . The rest were 104 Africans pupils ,how many Africans boy were in the computer class ?

I think you have a typo and you must mean that there were 104 African pupils, then

total number of boys --- x
total number of girls --- 121-x

(8/9)x + (4/5)(121-x) = 104
times 45 to clear fractions
40x + 36(121-x) =4680
40x + 4356 - 36x = 4680
4x = 324
x = 81 <---- total boys
and 40 total girls

So of the 81 total boys (8/9) of them or 72 were African

(and 32 of the 40 girls were African)
check:
40+81 = 121
72+32 = 104

Well, it seems like we have a number puzzle on our hands! Let's break it down. We know that 8/9 of the boys and 4/5 of the girls were African, and there were 104 African students in total.

Let's start with the boys. If 8/9 of them were African, that means there were 8 parts of African boys out of 9 total parts. So, we can set up a proportion:

8/9 = x/total number of boys

Cross-multiplying, we get:

9x = 8 * total number of boys

Similarly, for the girls, if 4/5 of them were African, that means there were 4 parts of African girls out of 5 total parts. So, we can set up another proportion:

4/5 = y/total number of girls

Cross-multiplying, we get:

5y = 4 * total number of girls

Now, we need to find the total number of boys and girls. Since 8/9 of the boys and 4/5 of the girls were African, we know that the remaining 1/9 of boys and 1/5 of girls were not African.

1/9 of boys not African = (1/9) * (total number of boys)
1/5 of girls not African = (1/5) * (total number of girls)

We can subtract these not African boys and girls from the total number of African students to get 104:

104 = (8/9) * (total number of boys) + (4/5) * (total number of girls) - (1/9) * (total number of boys) - (1/5) * (total number of girls)

Now, we have an equation with two variables. We need another equation to solve for both the total number of boys and girls. Unfortunately, we are missing some crucial information like the ratio of boys to girls in the class.

Without that information, it's impossible to determine how many African boys there were in the computer class. But hey, at least we got to have some fun with math, right?

Let's break down the information given step by step:

1. Total number of pupils in the computer class: 121.
2. Let's assume the number of boys in the class is B, and the number of girls is G. So, B + G = 121.

3. According to the given information, 8/9 of the boys were African. Let's calculate the number of African boys.
African boys = (8/9) * B

4. Also, 4/5 of the girls were African. Let's calculate the number of African girls.
African girls = (4/5) * G

5. The rest of the pupils, which is 104, were African. So, African boys + African girls = 104.

Now, we can create equations using the given information:

African boys + African girls = 104 (equation 1)
African boys = (8/9) * B (equation 2)
African girls = (4/5) * G (equation 3)
B + G = 121 (equation 4)

We need to solve these equations to find the number of African boys.

To find the number of African boys in the computer class, we need to follow these steps:

Step 1: Calculate the number of African students in the class.
We know that the total number of African students is 104, and they are comprised of 8/9 of the boys and 4/5 of the girls. To calculate the total number of African students, we can set up the following equation:

African students = (8/9) * Boys + (4/5) * Girls

Step 2: Calculate the number of boys and girls in the class.
We are given that there are 121 pupils in the class, and we know that the boys and girls make up the entire class. So we can set up the following equation:

Boys + Girls = 121

Step 3: Solve the system of equations.
We have two equations with two unknowns (Boys and Girls), and we can solve them simultaneously to find the values of Boys and Girls. Once we have these values, we can calculate the number of African boys using the formula from Step 1.

Let's solve the system of equations:

Equation 1: African students = (8/9) * Boys + (4/5) * Girls
Equation 2: Boys + Girls = 121

First, let's find the value of Boys:

Substituting the value of Boys from Equation 2 into Equation 1:
African students = (8/9) * (121 - Girls) + (4/5) * Girls

Simplifying Equation 1:
African students = (8/9) * 121 - (8/9) * Girls + (4/5) * Girls

Next, we'll solve for Girls by setting Equation 1 equal to 104 (since that's the given value for African students):
104 = (8/9) * 121 - (8/9) * Girls + (4/5) * Girls

Multiplying both sides by 9 to clear the fraction:
936 = 968 - 8 * Girls + 7 * Girls

Combining like terms:
936 = 968 - Girls

Subtracting 968 from both sides:
-32 = -Girls

Dividing both sides by -1:
32 = Girls

Now that we have the value for Girls, we can substitute it back into Equation 2 to find the value of Boys:
Boys + 32 = 121

Subtracting 32 from both sides:
Boys = 89

Finally, we can calculate the number of African boys using Equation 1:
African students = (8/9) * Boys + (4/5) * Girls
104 = (8/9) * 89 + (4/5) * 32

Calculating the values:
104 = 79.11 + 25.6

So, there are approximately 79 African boys in the computer class.