It's a similar triangle so the ratios of corresponding sides should be equal which means
BC/EF = AB/DE
21/14 = AB/10
AB = 21/14 * 10 = 15 cm
The fraction would be 997/1000, since this decimal goes to the thousandths place.
23/6
Now how many times does 6 go in 23 ???
The answer is 3 times
So, Remaining
= 23 - (6×3)
= 23- 18
= 5
So, the mixed fraction would be
= 3 5/6