Higher November 2020 Paper 6 Q8
8 Li has \(t\) toy bricks.
She only has red bricks and blue bricks.
Li picks two bricks, one after the other.
If the first brick she picks is red, the probability that the second brick is red is \(\frac{2}{3}\).
If the first brick she picks is blue, the probability that the second brick is red is \(\frac{7}{10}\).
Calculate the value of \(t\). [4]
| Answer | Marks | Part marks and guidance | |
|---|---|---|---|
| 31 nfww | 4 | M2 for 20 : 10 or \(\frac{20}{30}\) and 21 : 9 or \(\frac{21}{30}\) and SC1 for final answer 30 dep on M2 OR M1 for at least one other fraction equivalent to \(\frac{2}{3}\) seen, or one other fraction equivalent to \(\frac{7}{10}\) seen Alternative method using algebra M1 for \(\frac{r - 1}{t - 1} = \frac{2}{3}\) oe or \(\frac{r}{t - 1} = \frac{7}{10}\) oe M1 for \(3(r - 1) = 2(t - 1)\) and \(10r = 7(t - 1)\) or better M1 for elimination or substitution of \(r\) If 0 scored SC1 for answer 30 with no working | Notes on alternative method Where number of red = \(r\). Does not need to be defined. Accept any other letter. Implies first M1 mark A correct equation in \(b\) may imply first M1M1 marks |