M7MBA wrote: ↑

Thu Dec 17, 2020 11:22 am

Jack and Jill have a certain number of apples with them. The total number of apples with both of them is less than 80. If Jill gives a certain number of apples to Jack, then Jack will have four times the number of apples Jill has. If Jack gives the same number of apples to Jill, then Jack will have thrice the number of apples that Jill has. What can be the number of apples that Jack initially has?

A. 31
B. 36
C. 45
D. 62
E. 63

Answer: A

Solution:

Let K and L be the number of apples Jack and Jill have, respectively, and let a be the number of apples they give to each other. We can create the following equations and inequalities:

K + L < 80

4(L - a) = K + a

4L - 4a = K + a (Eq. 1)

3(L + a) = K - a

3L + 3a = K - a (Eq. 2)

If we subtract Eq. 2 from Eq. 1, we have:

L - 7a = 2a

L = 9a

If we add the two equations, we have:

7L - a = 2K

Substituting 9a for L, we have:

7(9a) - a = 2K

62a = 2K

31a = K

Substituting 31a for K and 9a for L in the inequality, we have:

31a + 9a < 80

40a < 80

a < 2

Since a must be a positive integer, a = 1, and therefore, K = 31a = 31 x 1 = 31.

Answer: A