Fred completed 10 math problems. this is 40%of the number of math problems he has to do. how many math problems does he have to do?

40 times 2 equals 80. So if we double both the numbers now he has done 20 math problems and 80% is done. But now we have to split the 10 in half because you cant do 80% plus 40% You have to do 80% plus 20% and that would be half of the 10. So in conclusion your answer is 25 math problems. ;)