9514 1404 393
Explanation:
a < b . . . . . given
0 < b -a . . . . . addition property of inequality
c < d . . . . . given
0 < d - c . . . . . addition property of inequality
0 < (b -a) +(d -c) . . . . addition is closed for positive real numbers
a +c < b +d . . . . . addition property of inequality
