Lisa and John are sister and brother. Lisa has as many brothers as sisters, and John has three times as many sisters as brothers. How many girls and boys are there in this family?

Step-by-step explanation:

What you have here is a system of two equations with unknown variables.

According to Lisa, her number of brothers and sisters is the same: B=S. This means that there is one more girls than boys: the amount of girls is Lisa + her sisters (imagine Lisa has 4 sisters and 4 brothers, than there are 5 girls and 4 boys). Thus, if G are the girls an B the boys:

G - 1 = B

According to John, the number of brothers is three times as many sisters as brothers. This implies that the number of brothers is the triple of the number of sisters. With B-1 being the number of John's brothers:

3 (B-1) = G

Resolving:

3 ((G-1) - 1) = G

3 (G-2) = G

3G - 6 = G

2G = 6

G=3

Replacing on our first equation:

B = 3 - 1

B=2

Answer: 3 girls and 2 boys