How can you know if this equation is true? 1/2+3/8=4/10=2/5

Well first you should know that 4/10 is simplified to 2/5 so that part is correct

But to know of the rest is correct, add the Fractions

so the common denominator will be 8 so 1/2 will become 4/8 + 3/8 = 7/8 but 7/8 does not equal 4/10