Complex sentence: This constituted a significant portion of the Mexican population in the US, with one estimate that 1/5th of Mexicans in California were repatriated by 1932, and 1/3rd of all Mexicans in the US between 1931 and 1934.
Split into simpler sentences: This constituted a significant portion of the Mexican population in the US. By one estimate, 1/5th of Mexicans in California were repatriated by 1932, and 1/3rd of all Mexicans in the US between 1931 and 1934.