CAT 2020 Slot 3QA Question 15

Number TheoryEasy

Let N, x and y be positive integers such that N = x + y, 2 < x < 10 and 14 < y < 23. If N > 25, then how many distinct values are possible for N?

Answer & solution

Answer: 6

Solution

Easy

List the integer ranges of xx and yy to get the maximum possible N=x+yN=x+y. Since NN can take every integer up to that maximum, just count the integers exceeding 2525.

1

Integer ranges. From $2 x{3,4,,9},xmax=9y{15,16,,22},ymax=22\begin{aligned} &x\in\{3,4,\dots,9\},\qquad x_{\max}=9\\ &y\in\{15,16,\dots,22\},\qquad y_{\max}=22 \end{aligned}

2

Maximum of NN.

Nmax=xmax+ymax=9+22=31\begin{aligned} &N_{\max}=x_{\max}+y_{\max}=9+22=31 \end{aligned}

Every integer from the minimum up to 3131 is attainable, so once N>25N>25 the possible values are the integers 2626 to 3131.

3

Count the values with N>25N>25.

N{26,27,28,29,30,31} count=6\begin{aligned} &N\in\{26,27,28,29,30,31\}\\ &\Rightarrow\ \text{count}=6 \end{aligned}
Number of distinct N=6\text{Number of distinct }N=6
CAT 2020 Slot 3 QA Q15: Let N, x and y be positive integers such that N = x + y, 2 < x < 10 and 14 < y < 23. If N > 25, then how many — Solution | TheCATExam