Round off the fractions! 3/7 rounds off (down) to 0 and 7/13 (being greater than 1/2) rounds up to 1. Then we have (24 + 0) / (3+1) = 24/4 = 6.
Unfortunately, this is "quite far" from the actual quotient, 6.9037.
We could experiment: Let 24 3/7 become 24 1/2 and 3 7/13 become 3 1/2.
(24 1/2) / (3 1/2) = (25/2) / (7/2) = 49/2 / (7/2) = 7 (which is "close" to the actual value, 6.9037.
Let's conclude that the estimation is between 6.9 and 7.0.