At first, David has 4 units.

Now, he has 3 units + 21 left.

If he gets back the 14 he gave away,

21 + 14 = 35

That means he had 3 units + 35 at first.

So 4 units = 3 units + 35.

Compare both sides, 1 unit = 35


Since James has 2 unit at first, 2 units x 35 = 70

Alternative method :


At first, James = 1u , David = 2u
Total = 3u

After giving 14 stamps to James,

James = 2u, David = 3u
Total = 5u

But realise that total number of stamps before giving and after giving, remains the same. So we need to make the number of units the same.


Total for before the giving : 3u x 5 = 15u
Total after 5u x 3 = 15u


So at first, James = 1u x 5 = 5u , David = 2u x 5 = 10u


After giving 14 stamps to James,

= 2u x 3 = 6u, David = 3u x 3 = 9u

Difference for both boys before and after
= 6u - 5u (or 10u - 9u)
= 1u

So 1u = 14

James at first = 5u = 5 x 14 = 70

Why theres 7x3 for David in the end? Isnt it only give 14.?

Because we are counting relative to James.

James now has 2 units + 14.
Which also = 2 x (1 unit + 7)

The idea is to divide the 14 equally among the 2 units. You create a bigger unit, (1unit + 7)

Now, James has 2/3 of the stamps David has.

So David now = 3 x (1 unit + 7)
= 3 units + 21


And notice that another way of counting David's stamps now is to subtract 14 stamps from what he had at first.

So now David now also = 4 units - 14


So 4 units - 14 = 3 units + 21

Since left hand side = right hand side,

4 units - 14 + 14 = 3 units + 21 + 14
4 units = 3 units + 35

Compare both side, 1 unit = 35