How to answer it
- Find the amount that doesn’t change: that’s the intercept.
- Find the amount added per unit: that’s the slope.
- Write f(x) = (slope)x + (intercept) and check one value.
Worked example
An original question in the SAT format, already answered.
SAT MathWorked example
A phone plan charges a fixed $15 each month plus $0.10 for each text message sent.
Which function gives the monthly cost C, in dollars, for t text messages?
- AC(t) = 15t + 0.10
- BC(t) = 15.10t
- CC(t) = 0.10(t + 15)
- DC(t) = 0.10t + 15
Answer: D. The $15 fee doesn’t change, so it’s the intercept, and $0.10 per text is the slope. Check: 100 texts cost 0.10(100) + 15 = $25. C works out to 0.10t + 1.5, which is wrong.
The most common trap
Swapping slope and intercept. Ask which number gets multiplied by the count.
Practice it
Hourly Prep’s daily sets mix linear functions with the rest of SAT Math, and send more of it your way when it’s a weak spot. See how the SAT prep plan works, or browse the other SAT skill guides.