Greetings from Brasil...
A - fraction addition/subtraction with equal denominators = keep the denominator and add/subtract only the numerators
B - fraction addition/subtraction with different denominators = we have to use LCM
2)
(10/15) - (7/15) - case A
(10 - 7)/15 = 3/15
3)
(8/10) + (3/5) - we can simplify the first fraction
(8/10) + (3/5) = (4/5) + (3/5) = (4 + 3)/5 = 7/5
4)
(3/4) - (2/5) - case B
LCM 4 and 5 = 20
(3/4) - (2/5) = 7/20