The length and width of the photograph will become 75% of the original dimensions. The original width is 12 inches. The reduced with is 75% of 12 inches. To find a percent of a number, multiply the percent by the number. Change the percent to a decimal by dividing the percent by 100, which means move the decimal of the percent two places to the left.
In the picture there are 6 triangles. Let's call the total amount x. Since 30% = 0.3 we can write 6 = 0.3x so x = 6 / 0.3 = 20. The picture that has 20 triangles is C.