Solution: Suppose water needs 0.7x kg, then cement needs x kg, yellow sand needs 2x kg and gravel needs 4.7xkg..

Equation: 0.7x+x+2x+4.7x=2 100.

Solution: 8.4x=2 100

x=250

So it needs: water 0.7x250 =175 kg; 250 kilograms of cement; 500 kilograms of yellow sand; Gravel1175kg