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48

In a football championship, 153 matches were played. Every two teams played one match with each other. The number of teams, participating in the championship is ............ .

Explanation

Let the number of team participating in championship be $n$.

Since, it is given that every two teams played one match with each other.

$\therefore$ Total match played $={ }^n C_2$

According to the question,

$$\begin{array}{r} & { }^n C_2 =153 \\ \Rightarrow & \frac{n(n-1)}{2} =153 \\ \Rightarrow & n^2-n =306 \\ \Rightarrow & n^2-n-306 =0 \\ \Rightarrow & (n-18)(n+17) =0 \\ \Rightarrow & n =18,-17 \quad \text{ [inadmissible]}\\ \therefore & n =18 \end{array}$$

49

The total number of ways in which six ' + ' and four '$-$' signs can be arranged in a line such that no two signs '$-$' occur together, is ............. .

Explanation

The arrangement can be understand with the help of following figure.

$-$ $+$ $-$ $+$ $-$ $+$ $-$ $+$ $-$ $+$ $-$ $+$ $-$

Thus, '+' sign can be arranged in 1 way because all are identical. and 4 negative signs can be arranged at 7 places in ${ }^7 C_4$ ways.

$$\begin{aligned} \therefore \quad \text { total number of ways } & ={ }^7 C_4 \times 1 \\ & =\frac{7!}{4!3!}=\frac{7 \times 6 \times 5 \times 4!}{3!\times 4!} \\ & =\frac{7 \times 6 \times 5}{3 \times 2 \times 1}=35 \text { ways } \end{aligned}$$

50

A box contains 2 white balls, 3 black balls and 4 red balls. The number of ways three balls be drawn from the box, if atleast one black ball is to be included in the draw is .............. .

Explanation

Since, there are 2 white, 3 black and 4 red balls. It is given that atleast one black ball is to be included in the draw.

$$\begin{aligned} \therefore \text { Required number of ways } & ={ }^3 \mathrm{C}_1 \times{ }^6 \mathrm{C}_2+{ }^3 \mathrm{C}_2 \times{ }^6 \mathrm{C}_1+{ }^3 \mathrm{C}_3 \\ & =3 \times 15+3 \times 6+1 \\ & =45+18+1=64 \end{aligned}$$

51

There are 12 points in a plane of which 5 points are collinear, then the number of lines obtained by joining these points in pairs is ${ }^{12} C_2-{ }^5 C_2$.

A
TRUE
B
FALSE
52

Three letters can be posted in five letter boxes in $3^5$ ways.

A
TRUE
B
FALSE