Answer:
Hey there!
a.
50=28.10+0.15m
Subtract 28.10
21.90=0.15m
Divide by 0.15
m=146
Jacob can send 146 messages.
b.
50=36.50+0.10m
Subtract 36.50
13.5=0.10m
Divide by 0.10
m=135
Roberto can send 135 messages.
Let me know if this helps :)
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