the best way to teach problem solving like the teacher has 15 student in his class and there are 3 more girls than boys how many girls than boys are there.
You have to learn how to "translate" the English into Math.
take it in pieces:
"there are 3 more girls than boys "
If we let the number of boys be x
then the above says there are x+3 girls
"the teacher has 15 student in his class"
or
the sum of the number of boys and girls is 15
x + x+3 = 15
2x = 15-3
2x = 12
x = 6
check: if there are 6 boys then there have to be 6+3 or 9 girls.
Are there 15 students in the class?
is 6+9 = 15 ?