What does 40 + 60 + 0.9 + 7.8 + 40.2 + 3.9 + 0.2 + -34 + -35 + -7 + 0.4 equal

A multiple of  

5

Explanation:

Percent means "per hundred", expressing a proportion.

The proportion of boys in the class is:

40

100

=

2

⋅

20

5

⋅

20

=

2

5

In other words, for every  

5

students in the class,  

2

are boys and  

3

are girls.

In order that the figure  

40

%

be exact, the number of students must be a multiple of  

5

.

Then the number of boys will be the corresponding multiple of  

2

and the number of girls the corresponding multiple of  

3

.

That is about as much as we can deduce from the question as given.