How much faster will a boy be growing than a girl during ten years

The girl is 10% taller than the boy now .
In ten years he will be 40% taller than he is now and 10% taller than the girl.

OK, reduce the givens to equations and manipulate them to get one variable

g1 = 1.1b1
b2 = 1.4b1
b2 = 1.1g2

b(rate) = (b2 - b1)/10
g(rate) = (g2 - g1)/10. Since it is not clear EXACTLY what the question is asking (i.e., the form of the answer, I'll give you both solutions

Absolute rate difference = (b2 - b1 - g2 + g1)/10 =
(1.4b1 - b1 - b2/1.1 + 1.1b1)/10 =
(1.4b1 - b1 - 1.4b1/1.1 + 1.1b1)/10 =
(1.54b1 -1.1b1 - 1.4b1 + 1.21b1)/11 = 0.24b1/11 = 0.218b1
Notice that the absolute rate difference depends on the start height of the boy (or the girl). It is apparent the boy was relatively rather small at time t(0)

Rate ratio = ((b2 - b1)/10)((g2 - g1)/10) = (b2 - b1)/(g2 - g1) =
(1.4b1 - b1)/(1.4b1/1.1 - 1.1 b1) =
0.4b1/(1.4b1 - 1.21b1/1.1) =
0.44/(1.54 - 1.21) =
0.44/0.33 =
4/3

The boy will be growing 33% faster than the girl