Answer:
8/40 + 20/40 = 28/40
Explanation:
It's about finding the common denominator for the fractions.
we can easily see that 40 is a common denominator for denominators 10 and 4. 20 would work too, but 40 is "instantly seen".
2/10 = (2*4) / (10*4) = 8/40
2/4 = (2*10) / (4*10) = 20/40
8/40 + 20/40 = (8+20) / 40 = 28/40