Teacher any easier way to workout the question pls?

Don't quite know if an easier method exists Jane, but the gist of this method is to understand the wording of the question. If the ratio between Mary and Henry is 3:1, that means Mary has 3 times more number of stamps than Henry. So, if Henry has 100 Stamps, Mary has 300. Which leads me to Equation 1 above, 3H = M, which means that 3 times Henry's stamps is equal to Mary's stamps. So in this case 3 × 100 = 300.

In the second part, they sell and buy some stamps. Mary gets 36 more, and Henry sells 20. Simply put, the stamps each of them have now are (M + 36) and (H - 20). Treat these two as separate terms. Now, Mary has got 36 stamps and Henry has lost 20. The ratio is now 5:1, which means Mary has 5 times more stamps. So again, imagine that Henry had 100 stamps and Mary had 500. 5 times Henry's stamps = Mary's stamps. Therefore, 5 × (H - 20) = M + 36. Remember from equation 1 that 'M' can be substituted with 3H and you're pretty much done :)

TQ for good explanation. I will try to understand it.
TQ for teaching

Okay! Please do let me know if you do not understand.