A contractor requires a cement mix that is 70% sand and 30% cement. He has 1400 kg of a cement mix that is 80% sand and 20% cement. How much cement should he add to this 1400 kg cement mix so that he gets the required mix?

added cement = 200 kg

Step-by-step explanation:

Sand = 0.8*1400

Sand = 1120 kg

If 1120 kg of sand represents the total sand in the 8:2 mixture.

We have to create the equation of the 7:3 mixture as follows.

The sand amount continues the same but the total amount of the mixture and cement will change.

0.7*mixture_mass + 0.3*mixture_mass = mixture_mass

By replacing values, we have:

1120 kg + 0.3 * (1400 kg + added cement) = 1400 kg + added cement

added cement = 200kg