A Riddle About the Twins

Hey I got a riddle for y’all.

In 1893, a family in a rural village welcomed twins. One a girl, and the other a boy. The girl was named Martha and the boy named Henry. As the children grew, their family’s fortune did as well, and soon they were able to move out of the rural village and into the city. It is now 1903, and the family of 4 purchases a large house and several acres of land on the edge of the city. Over the years, they start a farm. The father goes to town and purchases 2 cows, 8 chickens, 4 pigs, 2 horses, and $100 worth of plant seeds to fill up their farm. They build pens and stables for the animals, taking up half of their farm. They use the seeds to plant in the other half of the farm. Martha and her mother tend to the animals and Henry and his father manage the crops. The family becomes very well known for their farm, and begin supplying lots of food to their town. Martha and Henry are now going to school, where they make many friends. On Martha’s birthday, all her friends and family celebrate, with the exception of her now 18 year old brother, Henry, who has left home to fight in a war. How old is Martha?