Question 1121579
brother is b
Mary is at least 2b+5 as you had, and that is M >=2b+5, greater than OR equal to, and that is "at least"
if brother is 10, Mary is at least 25, 2(10)+5.  She may be older, but she has to be at least that.
The inequality is M >=+2b+5

Can Mary be 7?  Then 7 >=2b+5
2 >= 2b
2b <= 2
b <= 1
Yes.  She can be 7 if her brother is 1 or less than 1

Graphically, M >=2b +5.  The area in quadrant 1 to be shaded (possible) can be determined by checking the origin.  there, 0 >= 5, which is not true, so the upper part of the graph not involving the origin in Quadrant I represents the possibilities.  
{{{graph(300,300,-2,10,-5,20,2x+5)}}}