How do you do number 15 and 17? Help please?

15)

m = Miguel Indurian titles amount l = Lance Armstrong titles amount

they both won titles to the tour de france, if we add all together, they sum up to 12 for both of them
so.. whatever "m" and "l" are, we could say that m + l = 12

now... Miguel won 2 fewer than Lance, so, whatever "l" is, m = l - 2
thus [tex]\bf \begin{cases} \boxed{m}=l-2\\\\ m+l=12\\ ----------\\ \left( \boxed{l-2} \right)+l=12 \end{cases}[/tex]

solve for "l", to see how many Lance won

how many did Miguel win? well, m = l - 2

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17)

c = mgs amount in 12oz cup of regular coffee at starbucks
d = mgs amount in 12oz cup of regular decaffeinated coffee at starbucks

we know the regular coffee cup, has 13 times more than the decaf, same 12ouncer

thus whatever "c" and "d" are, we know that c = 13d
both 12ouncers put together, yield 280mgs of caffeine

so, one could say that whatever "c" and "d" are, c + d = 280

thus [tex]\bf \begin{cases} \boxed{c}=13d\\\\ c+d=280\\ ----------\\ \left( \boxed{13d}\right)+d=280 \end{cases}[/tex]

solve for "d" to see how much is in the decaffed cup

the regular coffee cup? well c = 13d