Bill and Melinda Gates have released their annual letter, and this time it is addressed to their “dear friend” Warren Buffett, who donated most of his fortune to the Bill and Melinda Gates Foundation in order to fight disease and reduce inequity.

In the 2017 letter, they reply to a letter Buffett sent them, asking the Gates’s to “reflect on what impact his gift has had on the world”.

Here are excerpts taken from the letter – you can also read the full letter here:

What follows is our answer to him.

It’s a story about the stunning gains the poorest people in the world have made over the last 25 years. This incredible progress has been made possible not only by the generosity of Warren and other philanthropists, the charitable giving of individuals across the world, and the efforts of the poor on their own behalf—but also by the huge contributions made by donor nations, which account for the vast majority of global health and development funding.

Our letter is being released amid dramatic political transitions in these countries, including new leadership in the United States and the United Kingdom. We hope this story will remind everyone why foreign aid should remain a priority—because by lifting up the poorest, we express the highest values of our nations.

One of the greatest of those values is the belief that the best investment any of us can ever make is in the lives of others. As we explain to Warren in our letter, the returns are tremendous.

Our Favorite Number

Bill: If we could show you only one number that proves how life has changed for the poorest, it would be 122 million—the number of children’s lives saved since 1990.

Melinda: Every September, the UN announces the number of children under five who died the previous year. Every year, this number breaks my heart and gives me hope. It’s tragic that so many children are dying, but every year more children live.

The Best Deal Is Vaccines

Melinda: And if you want to know the best deal within the deal—it’s vaccines. Coverage for the basic package of childhood vaccines is now the highest it’s ever been, at 86 percent. And the gap between the richest and the poorest countries is the lowest it’s ever been. Vaccines are the biggest reason for the drop in childhood deaths.

Reducing Newborn Mortality

Bill: Last year, about one million infants died on the day they were born. A total of more than 2.5 million died in their first month of life. As the total number of childhood deaths has dropped, the proportion that are newborn deaths has gone up. Newborn deaths now represent 45 percent of all childhood deaths, up from 40 percent in 1990.

Ending Malnutrition

Melinda: Here’s another challenging number that’s linked to the childhood mortality chart. Malnutrition is partly responsible for forty-five percent of childhood deaths.

Malnutrition is not starvation. Malnourished children can be getting enough calories, but not the right nutrients. That makes them more susceptible to conditions like pneumonia or diarrhea—and more likely to die from them.

But better nutrition is not just about preventing deaths.

The Power of Family Planning

Melinda: This is another number we follow closely. For the first time in history, more than 300 million women in developing countries are using modern methods of contraception. It took decades to reach 200 million women. It has taken only another 13 years to reach 300 million—and the impact in saving lives is fantastic.

Poverty Is Sexist

Bill: Poverty is sexist. The poorer the society, the less power women have. Men decide if a woman is allowed to go outside, talk to other women, earn income. Men decide if it’s acceptable to strike a woman. The male dominance in the poorest societies is mind-blowing.