Sandpit

Tip: Let's assume this is a right angled triangle with height $5$. How can we use the ratio $BX:XA=4:1$ to calculate the ratio $BY:YA$?

Tip: Let's assume the base is $1$ unit wide. How can we use this to calculate the area with height $XA$?

Tip: As the ratio links proportionally to the height of the triangle, so does the width.