For the answer to the question above,Let C-cost
Let n- the number of text messages
c=20+0.05n
c=10+0.10n
When cost is same;
20+0.05n=10+0.10n
20-10=0.10n-0.05n
10=0.05n
10/.05
n=200
Cost is same when 200 messages are sent.
I hope my answer helped you