A car that runs on corn ethanol will emit

MMt of CO2 each year.

2.256

Explanation:

This is a percent change problem. The CO2 emitted by a car using gas is 4.7 MMt per year. The CO2 emitted by a car using corn ethanol made from biomass is 52% less than 4.7 MMt.

Find 52% of 4.7 MMt:

0.52 * 4.7 = 2.444.

Subtract 2.444 MMt from 4.7 MMt:

4.7 - 2.444 = 2.256.

The solution is 2.256 MMt.