SAT Mathematics Practice Test 2

To find the atomic weight of an unknown element that is $20\%$ less than the atomic weight of calcium, multiply the atomic weight, in amu, of calcium by $\left(1-0.20\right):\left(40\right)\left(1-0.20\right)=\left(40\right)\left(0.8\right)=32.$
Choice A is incorrect. This value is 20% of the atomic weight of calcium, not an atomic weight 20% less than that atomic weight of calcium. Choice B is incorrect. This value is 20 amu less, not 20% less, than the atomic weight of calcium. Choice D is incorrect. This value is 20% more, not 20% less, than the atomic weight of calcium.