They have solved 2 sets of simultaneous equations and taken the average of the 4 equations:
y * x^25 =2100.....................(1)
y* x^35 =2800......................(2)
y* x^55 =5050.....................(3)
y* x^125 =40325.................(4)
When you solve these equations and take their average, you get x =~3% and y =~997.
