Question

7. If 20c3 + 20c4 + 21c5 = 22cr r =?

Answer 1

Answer:

37

Step-by-step explanation:

S=∑

r=1

20

(r−1)

20

C

r

=∑

r=1

20

20

C

r

−∑

r=1

20

20

C

r

=20∑

r=1

20

19

C

r−1

−(2

20

−1)

=20(2

19

)−(2

20

−1)

=1+(2

19

)(20−2)

=1+18(2

19

)

⇒p=18

⇒q=19

⇒p+q=37.