If you divide by a fraction, simply invert the numerator and denominator, and multiply. This means that if you divide by a (denominator) less than 1, you actually achieve a sum greater than the original numerator.
In this case, the divisor/denominator is 21/24, or 7/8. So the calculation is 4/7 times 8/7, or 32/49, ie 0.65.
There are very few common factors in the problem as stated, if the student is not familiar with converting fractions to decimals. It seems to me it would have been much simpler as 4/7 divided by 24/21, which would work out at 4/7 x 7/8, or 1/2. You will note this is greater than 4/7.