Can someone explain substitution more clearly?

as in solving systems by substitution. i'm genuinely stumped, can the process be explained in a coherent and straightforward manner with examples?

The sales totals at macy's food store have increased exponentially over the months. which of these best shows the sales in the first three months?

You can ride your bike around your block 6 times and the whole neighborhood 5 times in 16 minutes. you can ride your bike around your block 3 times and the whole neighborhood 10 times in 23 minutes. how long does it take you to ride around the neighborhood?

There are two major tests of readiness for college: the act and the sat. act scores are reported on a scale from 1 to 36. the distribution of act scores in recent years has been roughly normal with mean µ = 20.9 and standard deviation s = 4.8. sat scores are reported on a scale from 400 to 1600. sat scores have been roughly normal with mean µ = 1026 and standard deviation s = 209. reports on a student's act or sat usually give the percentile as well as the actual score. the percentile is just the cumulative proportion stated as a percent: the percent of all scores that were lower than this one. tanya scores 1260 on the sat. what is her percentile (± 1)?