compounded interest

End Week    Funds
1    4200.00
2    4410.00
3    4630.50
4    4862.03
5    5105.13
6    5360.38
7    5628.40
8    5909.82
9    6205.31
10    6515.58
11    6841.36
12    7183.43
13    7542.60
14    7919.73
15    8315.71
16    8731.50
17    9168.07
18    9626.48
19    10107.80
20    10613.19
21    11143.85
22    11701.04
23    12286.10
24    12900.40
25    13545.42
26    14222.69
27    14933.83
28    15680.52
29    16464.54
30    17287.77
31    18152.16
32    19059.77
33    20012.75

Most calculators, other than the most basic, have a button marked x^y (x superscript y) or similar. Assuming a starting balance of £4000 and a 5% increase compounded each week, to calculate your balance after y weeks enter the following:

4000 * 1.05 x^y y

If your calculator doesn't have a button to calculate x to the power y, I would recommend that you purchase a new calculator. You could alternatively use excel or an equivalent.