LCD of 8 and 5 is 40. 1/8=5/40 and 2/5=16/40 so their sum is (5+16)/40=21/40.
A rough estimate is obtained by observing that 1/8 is close to 1/10 and 2/5 is exactly 4/10. So the sum is 5/10=1/2. This is quite close to the exact result 21/40 because 21 is about half of 40.